Image encoding/decoding method and device using ibc prediction, and method for transmitting bitstream

ABSTRACT

An image encoding/decoding method and apparatus are provided. An image decoding method according to the present disclosure comprises determining whether a prediction mode of a current block is an intra block copy (IBC) mode based on information on the prediction mode of the current block, based on the prediction mode of the current block being an IBC mode, deriving motion information of the current block based on motion information of a corresponding block corresponding to the current block, and generating a prediction block for the current block, by performing IBC prediction based on the derived motion information of the current block. Based on absence of motion information of the corresponding block, the motion information of the current block is derived based on default motion information.

TECHNICAL FIELD

The present disclosure relates to an image encoding/decoding method andapparatus and, more particularly, to a method and apparatus forencoding/decoding an image using IBC prediction, and a method oftransmitting a bitstream generated by the image encodingmethod/apparatus of the present disclosure.

BACKGROUND ART

Recently, demand for high-resolution and high-quality images such ashigh definition (HD) images and ultra high definition (UHD) images isincreasing in various fields. As resolution and quality of image dataare improved, the amount of transmitted information or bits relativelyincreases as compared to existing image data. An increase in the amountof transmitted information or bits causes an increase in transmissioncost and storage cost.

Accordingly, there is a need for high-efficient image compressiontechnology for effectively transmitting, storing and reproducinginformation on high-resolution and high-quality images.

DISCLOSURE Technical Problem

An object of the present disclosure is to provide an imageencoding/decoding method and apparatus with improved encoding/decodingefficiency.

Another object of the present disclosure is to provide a method andapparatus for encoding/decoding an image using IBC prediction.

Another object of the present disclosure is to provide an imageencoding/decoding method and apparatus for performing IBC predictionafter deriving motion information of a current block for IBC predictionbased on motion information of a corresponding block or default motioninformation.

Another object of the present disclosure is to provide an imageencoding/decoding method and apparatus for performing IBC predictionafter deriving motion information of a chroma block for IBC predictionbased on motion information of a corresponding luma block or defaultmotion information.

Another object of the present disclosure is to provide a method oftransmitting a bitstream generated by an image encoding method orapparatus according to the present disclosure.

Another object of the present disclosure is to provide a recordingmedium storing a bitstream generated by an image encoding method orapparatus according to the present disclosure.

Another object of the present disclosure is to provide a recordingmedium storing a bitstream received, decoded and used to reconstruct animage by an image decoding apparatus according to the presentdisclosure.

The technical problems solved by the present disclosure are not limitedto the above technical problems and other technical problems which arenot described herein will become apparent to those skilled in the artfrom the following description.

Technical Solution

An image decoding method according to an aspect of the presentdisclosure may comprise determining whether a prediction mode of acurrent block is an intra block copy (IBC) mode based on information onthe prediction mode of the current block, based on the prediction modeof the current block being an IBC mode, deriving motion information ofthe current block based on motion information of a corresponding blockcorresponding to the current block, and generating a prediction blockfor the current block, by performing IBC prediction based on the derivedmotion information of the current block. Based on absence of motioninformation of the corresponding block, the motion information of thecurrent block is derived based on default motion information.

In the image decoding method of the present disclosure, the currentblock may be a chroma block and the corresponding block may be a lumablock, the deriving the motion information of the current block maycomprise partitioning the corresponding luma block into luma subblockshaving a predetermined size, and, for each of the luma subblocks,determining whether the luma subblock is a block encoded in IBC and,based on this, deriving motion information of a chroma subblockcorresponding to the luma subblock.

In the image decoding method of the present disclosure, based on theluma subblock being a block encoded in IBC, the motion information ofthe corresponding chroma subblock may be derived based on motioninformation of the luma subblock, and, based on the luma subblock beingnot a block encoded in IBC, the motion information of the correspondingchroma subblock may be derived based on the default motion information.

In the image decoding method of the present disclosure, the defaultmotion information may be derived based on a predetermined size of thesubblock or a size of the current chroma block.

In the image decoding method of the present disclosure, the defaultmotion information may be derived based on motion information at apredetermined position of the corresponding block.

In the image decoding method of the present disclosure, thepredetermined position of the corresponding block may be a centerposition or a top-left position of the corresponding block.

In the image decoding method of the present disclosure, the imagedecoding method may further comprise deriving the default motioninformation, the deriving the default motion information may comprisederiving a predetermined position of the corresponding block,determining whether the predetermined position of the correspondingblock has been encoded in IBC, based on the predetermined position ofthe corresponding block being encoded in IBC, deriving the defaultmotion information based on motion information of the predeterminedposition of the corresponding block; and, based on the predeterminedposition of the corresponding block being not encoded in IBC, derivingthe default motion information based on predetermined motioninformation.

In the image decoding method of the present disclosure, whether theprediction mode of the current block is an IBC mode may be determinedbased on whether a predetermined position of the corresponding block hasbeen encoded in IBC.

In the image decoding method of the present disclosure, based on thepredetermined position of the corresponding block being encoded in IBC,the prediction mode of the current block may be determined based on IBCperformance information obtained from a bitstream.

In the image decoding method of the present disclosure, based on thepredetermined position of the corresponding block being not encoded inIBC, the prediction mode of the current block may be determined to be amode other than the IBC mode.

An image decoding apparatus according to another aspect of the presentdisclosure may comprise a memory and at least one processor. The atleast one processor may determine whether a prediction mode of a currentblock is an intra block copy (IBC) mode based on information on theprediction mode of the current block, based on the prediction mode ofthe current block being an IBC mode, derive motion information of thecurrent block based on motion information of a corresponding blockcorresponding to the current block, and generate a prediction block forthe current block, by performing IBC prediction based on the derivedmotion information of the current block. Based on absence of motioninformation of the corresponding block, the motion information of thecurrent block may be derived based on default motion information.

An image encoding method according to another aspect of the presentdisclosure may comprise determining whether a prediction mode of acurrent block is an intra block copy (IBC) mode, based on the predictionmode of the current block being an IBC mode, deriving motion informationof the current block based on motion information of a correspondingblock corresponding to the current block, and generating a predictionblock for the current block, by performing IBC prediction based on thederived motion information of the current block. Based on absence ofmotion information of the corresponding block, the motion information ofthe current block may be derived based on default motion information.

In the image encoding method of the present disclosure, the currentblock may be a chroma block and the corresponding block may be a lumablock, the deriving the motion information of the current block maycomprise partitioning the corresponding luma block into luma subblockshaving a predetermined size, and, for each of the luma subblocks,determining whether the luma subblock is a block encoded in IBC and,based on this, deriving motion information of a chroma subblockcorresponding to the luma subblock.

In the image encoding method of the present disclosure, based on theluma subblock being a block encoded in IBC, the motion information ofthe corresponding chroma subblock may be derived based on motioninformation of the luma subblock, and, based on the luma subblock beingnot a block encoded in IBC, the motion information of the correspondingchroma subblock may be derived based on the default motion information.

In addition, a transmission method according to another aspect of thepresent disclosure may transmit a bitstream generated by the imageencoding apparatus or the image encoding method of the presentdisclosure.

In addition, a computer-readable recording medium according to anotheraspect of the present disclosure may store the bitstream generated bythe image encoding apparatus or the image encoding method of the presentdisclosure.

The features briefly summarized above with respect to the presentdisclosure are merely exemplary aspects of the detailed descriptionbelow of the present disclosure, and do not limit the scope of thepresent disclosure.

Advantageous Effects

According to the present disclosure, it is possible to provide an imageencoding/decoding method and apparatus with improved encoding/decodingefficiency.

According to the present disclosure, it is possible to provide a methodand apparatus for encoding/decoding an image using IBC prediction.

According to the present disclosure, it is possible to provide an imageencoding/decoding method and apparatus for performing IBC predictionafter deriving motion information of a current block for IBC predictionbased on motion information of a corresponding block or default motioninformation.

According to the present disclosure, it is possible to provide an imageencoding/decoding method and apparatus for performing IBC predictionafter deriving motion information of a chroma block for IBC predictionbased on motion information of a corresponding luma block or defaultmotion information.

Also, according to the present disclosure, it is possible to provide amethod of transmitting a bitstream generated by an image encoding methodor apparatus according to the present disclosure.

Also, according to the present disclosure, it is possible to provide arecording medium storing a bitstream generated by an image encodingmethod or apparatus according to the present disclosure.

Also, according to the present disclosure, it is possible to provide arecording medium storing a bitstream received, decoded and used toreconstruct an image by an image decoding apparatus according to thepresent disclosure.

It will be appreciated by persons skilled in the art that that theeffects that can be achieved through the present disclosure are notlimited to what has been particularly described hereinabove and otheradvantages of the present disclosure will be more clearly understoodfrom the detailed description.

DESCRIPTION OF DRAWINGS

FIG. 1 is a view schematically showing a video coding system, to whichan embodiment of the present disclosure is applicable.

FIG. 2 is a view schematically showing an image encoding apparatus, towhich an embodiment of the present disclosure is applicable.

FIG. 3 is a view schematically showing an image decoding apparatus, towhich an embodiment of the present disclosure is applicable.

FIG. 4 is a view showing a partitioning type of a block according to amulti-type tree structure;

FIG. 5 is a view showing a signaling mechanism of partition splittinginformation in a quadtree with nested multi-type tree structureaccording to the present disclosure

FIG. 6 is a flowchart illustrating an inter prediction based video/imageencoding method.

FIG. 7 is a view illustrating the configuration of an inter predictionunit 180 according to the present disclosure.

FIG. 8 is a flowchart illustrating an inter prediction based video/imagedecoding method.

FIG. 9 is a view illustrating the configuration of an inter predictionunit 260 according to the present disclosure.

FIG. 10 is a view illustrating neighboring blocks available as a spatialmerge candidate.

FIG. 11 is a view schematically illustrating a merge candidate listconstruction method according to an example of the present disclosure.

FIG. 12 is a view schematically illustrating a motion vector predictorcandidate list construction method according to an example of thepresent disclosure.

FIG. 13 is a view illustrating a syntax structure for transmitting MVDfrom an image encoding apparatus to an image decoding apparatusaccording to an example of the present disclosure.

FIG. 14 is a flowchart illustrating an IBC based video/image encodingmethod.

FIG. 15 is a view illustrating the configuration of a prediction unitfor performing an IBC based video/image encoding method according to thepresent disclosure.

FIG. 16 is a flowchart illustrating an IBC based video/image decodingmethod according to an embodiment.

FIG. 17 is a view illustrating a configuration of a prediction unit forperforming an IBC based video/image decoding method according to thepresent disclosure.

FIG. 18 is a flowchart illustrating a conventional method of derivingmotion information of a current chroma IBC block based on acorresponding luma block.

FIG. 19 is a view illustrating a luma block corresponding to a chromaIBC block in case of a dual tree.

FIG. 20 is a flowchart illustrating an embodiment of the presentdisclosure of deriving motion information of a current chroma IBC blockbased on a corresponding luma block.

FIG. 21 is a view illustrating a predetermined position of acorresponding luma block for deriving default motion information.

FIG. 22 is a flowchart illustrating an example of deriving defaultmotion information according to the present disclosure.

FIG. 23 is a flowchart illustrating a method of encoding a currentchroma block depending on whether default motion information is able tobe derived from a corresponding luma block.

FIG. 24 is a flowchart illustrating a method of decoding a currentchroma block depending on whether default motion information is able tobe derived from a corresponding luma block.

FIG. 25 is a flowchart illustrating an embodiment of the presentdisclosure of deriving motion information of a current block based on acorresponding block.

FIG. 26 is a view showing a content streaming system, to which anembodiment of the present disclosure is applicable.

MODE FOR INVENTION

Hereinafter, the embodiments of the present disclosure will be describedin detail with reference to the accompanying drawings so as to be easilyimplemented by those skilled in the art. However, the present disclosuremay be implemented in various different forms, and is not limited to theembodiments described herein.

In describing the present disclosure, if it is determined that thedetailed description of a related known function or construction rendersthe scope of the present disclosure unnecessarily ambiguous, thedetailed description thereof will be omitted. In the drawings, parts notrelated to the description of the present disclosure are omitted, andsimilar reference numerals are attached to similar parts.

In the present disclosure, when a component is “connected”, “coupled” or“linked” to another component, it may include not only a directconnection relationship but also an indirect connection relationship inwhich an intervening component is present. In addition, when a component“includes” or “has” other components, it means that other components maybe further included, rather than excluding other components unlessotherwise stated.

In the present disclosure, the terms first, second, etc. may be usedonly for the purpose of distinguishing one component from othercomponents, and do not limit the order or importance of the componentsunless otherwise stated. Accordingly, within the scope of the presentdisclosure, a first component in one embodiment may be referred to as asecond component in another embodiment, and similarly, a secondcomponent in one embodiment may be referred to as a first component inanother embodiment.

In the present disclosure, components that are distinguished from eachother are intended to clearly describe each feature, and do not meanthat the components are necessarily separated. That is, a plurality ofcomponents may be integrated and implemented in one hardware or softwareunit, or one component may be distributed and implemented in a pluralityof hardware or software units. Therefore, even if not stated otherwise,such embodiments in which the components are integrated or the componentis distributed are also included in the scope of the present disclosure.

In the present disclosure, the components described in variousembodiments do not necessarily mean essential components, and somecomponents may be optional components. Accordingly, an embodimentconsisting of a subset of components described in an embodiment is alsoincluded in the scope of the present disclosure. In addition,embodiments including other components in addition to componentsdescribed in the various embodiments are included in the scope of thepresent disclosure.

The present disclosure relates to encoding and decoding of an image, andterms used in the present disclosure may have a general meaning commonlyused in the technical field, to which the present disclosure belongs,unless newly defined in the present disclosure.

In the present disclosure, a “picture” generally refers to a unitrepresenting one image in a specific time period, and a slice/tile is acoding unit constituting a part of a picture, and one picture may becomposed of one or more slices/tiles. In addition, a slice/tile mayinclude one or more coding tree units (CTUs).

In the present disclosure, a “pixel” or a “pel” may mean a smallest unitconstituting one picture (or image). In addition, “sample” may be usedas a term corresponding to a pixel. A sample may generally represent apixel or a value of a pixel, and may represent only a pixel/pixel valueof a luma component or only a pixel/pixel value of a chroma component.

In the present disclosure, a “unit” may represent a basic unit of imageprocessing. The unit may include at least one of a specific region ofthe picture and information related to the region. The unit may be usedinterchangeably with terms such as “sample array”, “block” or “area” insome cases. In a general case, an MxN block may include samples (orsample arrays) or a set (or array) of transform coefficients of Mcolumns and N rows.

In the present disclosure, “current block” may mean one of “currentcoding block”, “current coding unit”, “coding target block”, “decodingtarget block” or “processing target block”. When prediction isperformed, “current block” may mean “current prediction block” or“prediction target block”. When transform (inversetransform)/quantization (dequantization) is performed, “current block”may mean “current transform block” or “transform target block”. Whenfiltering is performed, “current block” may mean “filtering targetblock”.

In addition, in the present disclosure, a “current block” may mean “aluma block of a current block” unless explicitly stated as a chromablock. The “chroma block of the current block” may be expressed byincluding an explicit description of a chroma block, such as “chromablock” or “current chroma block”.

In the present disclosure, the term “/” and “,” should be interpreted toindicate “and/or.” For instance, the expression “A/B” and “A, B” maymean “A and/or B.” Further, “A/B/C” and “A/B/C” may mean “at least oneof A, B, and/or C.”

In the present disclosure, the term “or” should be interpreted toindicate “and/or.” For instance, the expression “A or B” may comprise 1)only “A”, 2) only “B”, and/or 3) both “A and B”. In other words, in thepresent disclosure, the term “or” should be interpreted to indicate“additionally or alternatively.”

Overview of Video Coding System

FIG. 1 is a view showing a video coding system according to the presentdisclosure.

The video coding system according to an embodiment may include aencoding apparatus 10 and a decoding apparatus 20. The encodingapparatus 10 may deliver encoded video and/or image information or datato the decoding apparatus 20 in the form of a file or streaming via adigital storage medium or network.

The encoding apparatus 10 according to an embodiment may include a videosource generator 11, an encoding unit 12 and a transmitter 13. Thedecoding apparatus 20 according to an embodiment may include a receiver21, a decoding unit 22 and a renderer 23. The encoding unit 12 may becalled a video/image encoding unit, and the decoding unit 22 may becalled a video/image decoding unit. The transmitter 13 may be includedin the encoding unit 12. The receiver 21 may be included in the decodingunit 22. The renderer 23 may include a display and the display may beconfigured as a separate device or an external component.

The video source generator 11 may acquire a video/image through aprocess of capturing, synthesizing or generating the video/image. Thevideo source generator 11 may include a video/image capture deviceand/or a video/image generating device. The video/image capture devicemay include, for example, one or more cameras, video/image archivesincluding previously captured video/images, and the like. Thevideo/image generating device may include, for example, computers,tablets and smartphones, and may (electronically) generate video/images.For example, a virtual video/image may be generated through a computeror the like. In this case, the video/image capturing process may bereplaced by a process of generating related data.

The encoding unit 12 may encode an input video/image. The encoding unit12 may perform a series of procedures such as prediction, transform, andquantization for compression and coding efficiency. The encoding unit 12may output encoded data (encoded video/image information) in the form ofa bitstream.

The transmitter 13 may transmit the encoded video/image information ordata output in the form of a bitstream to the receiver 21 of thedecoding apparatus 20 through a digital storage medium or a network inthe form of a file or streaming The digital storage medium may includevarious storage mediums such as USB, SD, CD, DVD, Blu-ray, HDD, SSD, andthe like. The transmitter 13 may include an element for generating amedia file through a predetermined file format and may include anelement for transmission through a broadcast/communication network. Thereceiver 21 may extract/receive the bitstream from the storage medium ornetwork and transmit the bitstream to the decoding unit 22.

The decoding unit 22 may decode the video/image by performing a seriesof procedures such as dequantization, inverse transform, and predictioncorresponding to the operation of the encoding unit 12.

The renderer 23 may render the decoded video/image. The renderedvideo/image may be displayed through the display.

Overview of Image Encoding Apparatus

FIG. 2 is a view schematically showing an image encoding apparatus, towhich an embodiment of the present disclosure is applicable.

As shown in FIG. 2, the image encoding apparatus 100 may include animage partitioner 110, a subtractor 115, a transformer 120, a quantizer130, a dequantizer 140, an inverse transformer 150, an adder 155, afilter 160, a memory 170, an inter prediction unit 180, an intraprediction unit 185 and an entropy encoder 190. The inter predictionunit 180 and the intra prediction unit 185 may be collectively referredto as a “prediction unit”. The transformer 120, the quantizer 130, thedequantizer 140 and the inverse transformer 150 may be included in aresidual processor. The residual processor may further include thesubtractor 115.

All or at least some of the plurality of components configuring theimage encoding apparatus 100 may be configured by one hardware component(e.g., an encoder or a processor) in some embodiments. In addition, thememory 170 may include a decoded picture buffer (DPB) and may beconfigured by a digital storage medium.

The image partitioner 110 may partition an input image (or a picture ora frame) input to the image encoding apparatus 100 into one or moreprocessing units. For example, the processing unit may be called acoding unit (CU). The coding unit may be acquired by recursivelypartitioning a coding tree unit (CTU) or a largest coding unit (LCU)according to a quad-tree binary-tree ternary-tree (QT/BT/TT) structure.For example, one coding unit may be partitioned into a plurality ofcoding units of a deeper depth based on a quad tree structure, a binarytree structure, and/or a ternary structure. For partitioning of thecoding unit, a quad tree structure may be applied first and the binarytree structure and/or ternary structure may be applied later. The codingprocedure according to the present disclosure may be performed based onthe final coding unit that is no longer partitioned. The largest codingunit may be used as the final coding unit or the coding unit of deeperdepth acquired by partitioning the largest coding unit may be used asthe final coding unit. Here, the coding procedure may include aprocedure of prediction, transform, and reconstruction, which will bedescribed later. As another example, the processing unit of the codingprocedure may be a prediction unit (PU) or a transform unit (TU). Theprediction unit and the transform unit may be split or partitioned fromthe final coding unit. The prediction unit may be a unit of sampleprediction, and the transform unit may be a unit for deriving atransform coefficient and/or a unit for deriving a residual signal fromthe transform coefficient.

The prediction unit (the inter prediction unit 180 or the intraprediction unit 185) may perform prediction on a block to be processed(current block) and generate a predicted block including predictionsamples for the current block. The prediction unit may determine whetherintra prediction or inter prediction is applied on a current block or CUbasis. The prediction unit may generate various information related toprediction of the current block and transmit the generated informationto the entropy encoder 190. The information on the prediction may beencoded in the entropy encoder 190 and output in the form of abitstream.

The intra prediction unit 185 may predict the current block by referringto the samples in the current picture. The referred samples may belocated in the neighborhood of the current block or may be located apartaccording to the intra prediction mode and/or the intra predictiontechnique. The intra prediction modes may include a plurality ofnon-directional modes and a plurality of directional modes. Thenon-directional mode may include, for example, a DC mode and a planarmode. The directional mode may include, for example, 33 directionalprediction modes or 65 directional prediction modes according to thedegree of detail of the prediction direction. However, this is merely anexample, more or less directional prediction modes may be used dependingon a setting. The intra prediction unit 185 may determine the predictionmode applied to the current block by using a prediction mode applied toa neighboring block.

The inter prediction unit 180 may derive a predicted block for thecurrent block based on a reference block (reference sample array)specified by a motion vector on a reference picture. In this case, inorder to reduce the amount of motion information transmitted in theinter prediction mode, the motion information may be predicted in unitsof blocks, subblocks, or samples based on correlation of motioninformation between the neighboring block and the current block. Themotion information may include a motion vector and a reference pictureindex. The motion information may further include inter predictiondirection (L0 prediction, L1 prediction, Bi prediction, etc.)information. In the case of inter prediction, the neighboring block mayinclude a spatial neighboring block present in the current picture and atemporal neighboring block present in the reference picture. Thereference picture including the reference block and the referencepicture including the temporal neighboring block may be the same ordifferent. The temporal neighboring block may be called a collocatedreference block, a co-located CU (colCU), and the like. The referencepicture including the temporal neighboring block may be called acollocated picture (colPic). For example, the inter prediction unit 180may configure a motion information candidate list based on neighboringblocks and generate information indicating which candidate is used toderive a motion vector and/or a reference picture index of the currentblock. Inter prediction may be performed based on various predictionmodes. For example, in the case of a skip mode and a merge mode, theinter prediction unit 180 may use motion information of the neighboringblock as motion information of the current block. In the case of theskip mode, unlike the merge mode, the residual signal may not betransmitted. In the case of the motion vector prediction (MVP) mode, themotion vector of the neighboring block may be used as a motion vectorpredictor, and the motion vector of the current block may be signaled byencoding a motion vector difference and an indicator for a motion vectorpredictor. The motion vector difference may mean a difference betweenthe motion vector of the current block and the motion vector predictor.

The prediction unit may generate a prediction signal based on variousprediction methods and prediction techniques described below. Forexample, the prediction unit may not only apply intra prediction orinter prediction but also simultaneously apply both intra prediction andinter prediction, in order to predict the current block. A predictionmethod of simultaneously applying both intra prediction and interprediction for prediction of the current block may be called combinedinter and intra prediction (CIIP). In addition, the prediction unit mayperform intra block copy (IBC) for prediction of the current block.Intra block copy may be used for content image/video coding of a game orthe like, for example, screen content coding (SCC). IBC is a method ofpredicting a current picture using a previously reconstructed referenceblock in the current picture at a location apart from the current blockby a predetermined distance. When IBC is applied, the location of thereference block in the current picture may be encoded as a vector (blockvector) corresponding to the predetermined distance. IBC basicallyperforms prediction in the current picture, but may be performedsimilarly to inter prediction in that a reference block is derivedwithin the current picture. That is, IBC may use at least one of theinter prediction techniques described in the present disclosure.

The prediction signal generated by the prediction unit may be used togenerate a reconstructed signal or to generate a residual signal. Thesubtractor 115 may generate a residual signal (residual block orresidual sample array) by subtracting the prediction signal (predictedblock or prediction sample array) output from the prediction unit fromthe input image signal (original block or original sample array). Thegenerated residual signal may be transmitted to the transformer 120.

The transformer 120 may generate transform coefficients by applying atransform technique to the residual signal. For example, the transformtechnique may include at least one of a discrete cosine transform (DCT),a discrete sine transform (DST), a karhunen-loeve transform (KLT), agraph-based transform (GBT), or a conditionally non-linear transform(CNT). Here, the GBT means transform obtained from a graph whenrelationship information between pixels is represented by the graph. TheCNT refers to transform acquired based on a prediction signal generatedusing all previously reconstructed pixels. In addition, the transformprocess may be applied to square pixel blocks having the same size ormay be applied to blocks having a variable size rather than square.

The quantizer 130 may quantize the transform coefficients and transmitthem to the entropy encoder 190. The entropy encoder 190 may encode thequantized signal (information on the quantized transform coefficients)and output a bitstream. The information on the quantized transformcoefficients may be referred to as residual information. The quantizer130 may rearrange quantized transform coefficients in a block form intoa one-dimensional vector form based on a coefficient scanning order andgenerate information on the quantized transform coefficients based onthe quantized transform coefficients in the one-dimensional vector form.

The entropy encoder 190 may perform various encoding methods such as,for example, exponential Golomb, context-adaptive variable length coding(CAVLC), context-adaptive binary arithmetic coding (CABAC), and thelike. The entropy encoder 190 may encode information necessary forvideo/image reconstruction other than quantized transform coefficients(e.g., values of syntax elements, etc.) together or separately. Encodedinformation (e.g., encoded video/image information) may be transmittedor stored in units of network abstraction layers (NALs) in the form of abitstream. The video/image information may further include informationon various parameter sets such as an adaptation parameter set (APS), apicture parameter set (PPS), a sequence parameter set (SPS), or a videoparameter set (VPS). In addition, the video/image information mayfurther include general constraint information. The signaledinformation, transmitted information and/or syntax elements described inthe present disclosure may be encoded through the above-describedencoding procedure and included in the bitstream.

The bitstream may be transmitted over a network or may be stored in adigital storage medium. The network may include a broadcasting networkand/or a communication network, and the digital storage medium mayinclude various storage media such as USB, SD, CD, DVD, Blu-ray, HDD,SSD, and the like. A transmitter (not shown) transmitting a signaloutput from the entropy encoder 190 and/or a storage unit (not shown)storing the signal may be included as internal/external element of theimage encoding apparatus 100. Alternatively, the transmitter may beprovided as the component of the entropy encoder 190.

The quantized transform coefficients output from the quantizer 130 maybe used to generate a residual signal. For example, the residual signal(residual block or residual samples) may be reconstructed by applyingdequantization and inverse transform to the quantized transformcoefficients through the dequantizer 140 and the inverse transformer150.

The adder 155 adds the reconstructed residual signal to the predictionsignal output from the inter prediction unit 180 or the intra predictionunit 185 to generate a reconstructed signal (reconstructed picture,reconstructed block, reconstructed sample array). If there is noresidual for the block to be processed, such as a case where the skipmode is applied, the predicted block may be used as the reconstructedblock. The adder 155 may be called a reconstructor or a reconstructedblock generator. The generated reconstructed signal may be used forintra prediction of a next block to be processed in the current pictureand may be used for inter prediction of a next picture through filteringas described below.

The filter 160 may improve subjective/objective image quality byapplying filtering to the reconstructed signal. For example, the filter160 may generate a modified reconstructed picture by applying variousfiltering methods to the reconstructed picture and store the modifiedreconstructed picture in the memory 170, specifically, a DPB of thememory 170. The various filtering methods may include, for example,deblocking filtering, a sample adaptive offset, an adaptive loop filter,a bilateral filter, and the like. The filter 160 may generate variousinformation related to filtering and transmit the generated informationto the entropy encoder 190 as described later in the description of eachfiltering method. The information related to filtering may be encoded bythe entropy encoder 190 and output in the form of a bitstream.

The modified reconstructed picture transmitted to the memory 170 may beused as the reference picture in the inter prediction unit 180. Wheninter prediction is applied through the image encoding apparatus 100,prediction mismatch between the image encoding apparatus 100 and theimage decoding apparatus may be avoided and encoding efficiency may beimproved.

The DPB of the memory 170 may store the modified reconstructed picturefor use as a reference picture in the inter prediction unit 180. Thememory 170 may store the motion information of the block from which themotion information in the current picture is derived (or encoded) and/orthe motion information of the blocks in the picture that have alreadybeen reconstructed. The stored motion information may be transmitted tothe inter prediction unit 180 and used as the motion information of thespatial neighboring block or the motion information of the temporalneighboring block. The memory 170 may store reconstructed samples ofreconstructed blocks in the current picture and may transfer thereconstructed samples to the intra prediction unit 185.

Overview of Image Decoding Apparatus

FIG. 3 is a view schematically showing an image decoding apparatus, towhich an embodiment of the present disclosure is applicable.

As shown in FIG. 3, the image decoding apparatus 200 may include anentropy decoder 210, a dequantizer 220, an inverse transformer 230, anadder 235, a filter 240, a memory 250, an inter prediction unit 260 andan intra prediction unit 265. The inter prediction unit 260 and theintra prediction unit 265 may be collectively referred to as a“prediction unit”. The dequantizer 220 and the inverse transformer 230may be included in a residual processor.

All or at least some of a plurality of components configuring the imagedecoding apparatus 200 may be configured by a hardware component (e.g.,a decoder or a processor) according to an embodiment. In addition, thememory 250 may include a decoded picture buffer (DPB) or may beconfigured by a digital storage medium.

The image decoding apparatus 200, which has received a bitstreamincluding video/image information, may reconstruct an image byperforming a process corresponding to a process performed by the imageencoding apparatus 100 of FIG. 2. For example, the image decodingapparatus 200 may perform decoding using a processing unit applied inthe image encoding apparatus. Thus, the processing unit of decoding maybe a coding unit, for example. The coding unit may be acquired bypartitioning a coding tree unit or a largest coding unit. Thereconstructed image signal decoded and output through the image decodingapparatus 200 may be reproduced through a reproducing apparatus (notshown).

The image decoding apparatus 200 may receive a signal output from theimage encoding apparatus of FIG. 2 in the form of a bitstream. Thereceived signal may be decoded through the entropy decoder 210. Forexample, the entropy decoder 210 may parse the bitstream to deriveinformation (e.g., video/image information) necessary for imagereconstruction (or picture reconstruction). The video/image informationmay further include information on various parameter sets such as anadaptation parameter set (APS), a picture parameter set (PPS), asequence parameter set (SPS), or a video parameter set (VPS). Inaddition, the video/image information may further include generalconstraint information. The image decoding apparatus may further decodepicture based on the information on the parameter set and/or the generalconstraint information. Signaled/received information and/or syntaxelements described in the present disclosure may be decoded through thedecoding procedure and obtained from the bitstream. For example, theentropy decoder 210 decodes the information in the bitstream based on acoding method such as exponential Golomb coding, CAVLC, or CABAC, andoutput values of syntax elements required for image reconstruction andquantized values of transform coefficients for residual. Morespecifically, the CABAC entropy decoding method may receive a bincorresponding to each syntax element in the bitstream, determine acontext model using a decoding target syntax element information,decoding information of a neighboring block and a decoding target blockor information of a symbol/bin decoded in a previous stage, and performarithmetic decoding on the bin by predicting a probability of occurrenceof a bin according to the determined context model, and generate asymbol corresponding to the value of each syntax element. In this case,the CABAC entropy decoding method may update the context model by usingthe information of the decoded symbol/bin for a context model of a nextsymbol/bin after determining the context model. The information relatedto the prediction among the information decoded by the entropy decoder210 may be provided to the prediction unit (the inter prediction unit260 and the intra prediction unit 265), and the residual value on whichthe entropy decoding was performed in the entropy decoder 210, that is,the quantized transform coefficients and related parameter information,may be input to the dequantizer 220. In addition, information onfiltering among information decoded by the entropy decoder 210 may beprovided to the filter 240. Meanwhile, a receiver (not shown) forreceiving a signal output from the image encoding apparatus may befurther configured as an internal/external element of the image decodingapparatus 200, or the receiver may be a component of the entropy decoder210.

Meanwhile, the image decoding apparatus according to the presentdisclosure may be referred to as a video/image/picture decodingapparatus. The image decoding apparatus may be classified into aninformation decoder (video/image/picture information decoder) and asample decoder (video/image/picture sample decoder). The informationdecoder may include the entropy decoder 210. The sample decoder mayinclude at least one of the dequantizer 220, the inverse transformer230, the adder 235, the filter 240, the memory 250, the inter predictionunit 260 or the intra prediction unit 265.

The dequantizer 220 may dequantize the quantized transform coefficientsand output the transform coefficients. The dequantizer 220 may rearrangethe quantized transform coefficients in the form of a two-dimensionalblock. In this case, the rearrangement may be performed based on thecoefficient scanning order performed in the image encoding apparatus.The dequantizer 220 may perform dequantization on the quantizedtransform coefficients by using a quantization parameter (e.g.,quantization step size information) and obtain transform coefficients.

The inverse transformer 230 may inversely transform the transformcoefficients to obtain a residual signal (residual block, residualsample array).

The prediction unit may perform prediction on the current block andgenerate a predicted block including prediction samples for the currentblock. The prediction unit may determine whether intra prediction orinter prediction is applied to the current block based on theinformation on the prediction output from the entropy decoder 210 andmay determine a specific intra/inter prediction mode (predictiontechnique).

It is the same as described in the prediction unit of the image encodingapparatus 100 that the prediction unit may generate the predictionsignal based on various prediction methods (techniques) which will bedescribed later.

The intra prediction unit 265 may predict the current block by referringto the samples in the current picture. The description of the intraprediction unit 185 is equally applied to the intra prediction unit 265.

The inter prediction unit 260 may derive a predicted block for thecurrent block based on a reference block (reference sample array)specified by a motion vector on a reference picture. In this case, inorder to reduce the amount of motion information transmitted in theinter prediction mode, motion information may be predicted in units ofblocks, subblocks, or samples based on correlation of motion informationbetween the neighboring block and the current block. The motioninformation may include a motion vector and a reference picture index.The motion information may further include inter prediction direction(L0 prediction, L1 prediction, Bi prediction, etc.) information. In thecase of inter prediction, the neighboring block may include a spatialneighboring block present in the current picture and a temporalneighboring block present in the reference picture. For example, theinter prediction unit 260 may configure a motion information candidatelist based on neighboring blocks and derive a motion vector of thecurrent block and/or a reference picture index based on the receivedcandidate selection information. Inter prediction may be performed basedon various prediction modes, and the information on the prediction mayinclude information indicating a mode of inter prediction for thecurrent block.

The adder 235 may generate a reconstructed signal (reconstructedpicture, reconstructed block, reconstructed sample array) by adding theobtained residual signal to the prediction signal (predicted block,predicted sample array) output from the prediction unit (including theinter prediction unit 260 and/or the intra prediction unit 265). Ifthere is no residual for the block to be processed, such as when theskip mode is applied, the predicted block may be used as thereconstructed block. The description of the adder 155 is equallyapplicable to the adder 235. The adder 235 may be called a reconstructoror a reconstructed block generator. The generated reconstructed signalmay be used for intra prediction of a next block to be processed in thecurrent picture and may be used for inter prediction of a next picturethrough filtering as described below.

The filter 240 may improve subjective/objective image quality byapplying filtering to the reconstructed signal. For example, the filter240 may generate a modified reconstructed picture by applying variousfiltering methods to the reconstructed picture and store the modifiedreconstructed picture in the memory 250, specifically, a DPB of thememory 250. The various filtering methods may include, for example,deblocking filtering, a sample adaptive offset, an adaptive loop filter,a bilateral filter, and the like.

The (modified) reconstructed picture stored in the DPB of the memory 250may be used as a reference picture in the inter prediction unit 260. Thememory 250 may store the motion information of the block from which themotion information in the current picture is derived (or decoded) and/orthe motion information of the blocks in the picture that have alreadybeen reconstructed. The stored motion information may be transmitted tothe inter prediction unit 260 so as to be utilized as the motioninformation of the spatial neighboring block or the motion informationof the temporal neighboring block. The memory 250 may storereconstructed samples of reconstructed blocks in the current picture andtransfer the reconstructed samples to the intra prediction unit 265.

In the present disclosure, the embodiments described in the filter 160,the inter prediction unit 180, and the intra prediction unit 185 of theimage encoding apparatus 100 may be equally or correspondingly appliedto the filter 240, the inter prediction unit 260, and the intraprediction unit 265 of the image decoding apparatus 200.

Overview of Partitioning of CTU

As described above, the coding unit may be acquired by recursivelypartitioning the coding tree unit (CTU) or the largest coding unit (LCU)according to a quad-tree/binary-tree/ternary-tree (QT/BT/TT) structure.For example, the CTU may be first partitioned into quadtree structures.Thereafter, leaf nodes of the quadtree structure may be furtherpartitioned by a multi-type tree structure.

Partitioning according to quadtree means that a current CU (or CTU) ispartitioned into equally four. By partitioning according to quadtree,the current CU may be partitioned into four CUs having the same widthand the same height. When the current CU is no longer partitioned intothe quadtree structure, the current CU corresponds to the leaf node ofthe quad-tree structure. The CU corresponding to the leaf node of thequadtree structure may be no longer partitioned and may be used as theabove-described final coding unit. Alternatively, the CU correspondingto the leaf node of the quadtree structure may be further partitioned bya multi-type tree structure.

FIG. 4 is a view showing an embodiment of a partitioning type of a blockaccording to a multi-type tree structure. Partitioning according to themulti-type tree structure may include two types of splitting accordingto a binary tree structure and two types of splitting according to aternary tree structure.

The two types of splitting according to the binary tree structure mayinclude vertical binary splitting (SPLIT_BT_VER) and horizontal binarysplitting (SPLIT_BT_HOR). Vertical binary splitting (SPLIT_BT_VER) meansthat the current CU is split into equally two in the vertical direction.As shown in FIG. 4, by vertical binary splitting, two CUs having thesame height as the current CU and having a width which is half the widthof the current CU may be generated. Horizontal binary splitting(SPLIT_BT_HOR) means that the current CU is split into equally two inthe horizontal direction. As shown in FIG. 5, by horizontal binarysplitting, two CUs having a height which is half the height of thecurrent CU and having the same width as the current CU may be generated.

Two types of splitting according to the ternary tree structure mayinclude vertical ternary splitting (SPLIT_TT_VER) and horizontal ternarysplitting (SPLIT_TT_HOR). In vertical ternary splitting (SPLIT_TT_VER),the current CU is split in the vertical direction at a ratio of 1:2:1.As shown in FIG. 4, by vertical ternary splitting, two CUs having thesame height as the current CU and having a width which is ¼ of the widthof the current CU and a CU having the same height as the current CU andhaving a width which is half the width of the current CU may begenerated. In horizontal ternary splitting (SPLIT_TT_HOR), the currentCU is split in the horizontal direction at a ratio of 1:2:1. As shown inFIG. 4, by horizontal ternary splitting, two CUs having a height whichis ¼ of the height of the current CU and having the same width as thecurrent CU and a CU having a height which is half the height of thecurrent CU and having the same width as the current CU may be generated.

FIG. 5 is a view showing a signaling mechanism of partition splittinginformation in a quadtree with nested multi-type tree structureaccording to the present disclosure.

Here, the CTU is treated as the root node of the quadtree, and ispartitioned for the first time into a quadtree structure. Information(e.g., qt_split_flag) indicating whether quadtree splitting is performedwith respect to the current CU (CTU or node (QT_node) of the quadtree)is signaled. For example, when qt_split_flag has a first value (e.g.,“1”), the current CU may be quadtree-partitioned. In addition, whenqt_split_flag has a second value (e.g., “0”), the current CU is notquadtree-partitioned, but becomes the leaf node (QT_leaf_node) of thequadtree. Each quadtree leaf node may then be further partitioned intomultitype tree structures. That is, the leaf node of the quadtree maybecome the node (MTT_node) of the multi-type tree. In the multitype treestructure, a first flag (e.g., Mtt_split_cu_flag) is signaled toindicate whether the current node is additionally partitioned. If thecorresponding node is additionally partitioned (e.g., if the first flagis 1), a second flag (e.g., Mtt_split_cu_vertical_flag) may be signaledto indicate the splitting direction. For example, the splittingdirection may be a vertical direction if the second flag is 1 and may bea horizontal direction if the second flag is 0. Then, a third flag(e.g., Mtt_split_cu_binary_flag) may be signaled to indicate whether thesplit type is a binary split type or a ternary split type. For example,the split type may be a binary split type when the third flag is 1 andmay be a ternary split type when the third flag is 0. The node of themulti-type tree acquired by binary splitting or ternary splitting may befurther partitioned into multi-type tree structures. However, the nodeof the multi-type tree may not be partitioned into quadtree structures.If the first flag is 0, the corresponding node of the multi-type tree isno longer split but becomes the leaf node (MTT_leaf_node) of themulti-type tree. The CU corresponding to the leaf node of the multi-typetree may be used as the above-described final coding unit.

Based on the mtt_split_cu_vertical_flag and themtt_split_cu_binary_flag, a multi-type tree splitting mode(MttSplitMode) of a CU may be derived as shown in Table 1 below. In thefollowing description, the multi-type tree splitting mode may bereferred to as a multi-tree splitting type or splitting type.

TABLE 1 MttSplitMode mtt_split_cu_vertical_flag mtt_split_cu_binary_flagSPLIT_TT_HOR 0 0 SPLIT_BT_HOR 0 1 SPLIT_TT_VER 1 0 SPLIT_BT_VER 1 1

One CTU may include a coding block of luma samples (hereinafter referredto as a “luma block”) and two coding blocks of chroma samplescorresponding thereto (hereinafter referred to as “chroma blocks”). Theabove-described coding tree scheme may be equally or separately appliedto the luma block and chroma block of the current CU. Specifically, theluma and chroma blocks in one CTU may be partitioned into the same blocktree structure and, in this case, the tree structure is represented asSINGLE_TREE. Alternatively, the luma and chroma blocks in one CTU may bepartitioned into separate block tree structures, and, in this case, thetree structure may be represented as DUAL_TREE. That is, when the CTU ispartitioned into dual trees, the block tree structure for the luma blockand the block tree structure for the chroma block may be separatelypresent. In this case, the block tree structure for the luma block maybe called DUAL_TREE_LUMA, and the block tree structure for the chromacomponent may be called DUAL_TREE_CHROMA. For P and B slice/tile groups,luma and chroma blocks in one CTU may be limited to have the same codingtree structure. However, for I slice/tile groups, luma and chroma blocksmay have a separate block tree structure from each other. If theseparate block tree structure is applied, the luma CTB may bepartitioned into CUs based on a particular coding tree structure, andthe chroma CTB may be partitioned into chroma CUs based on anothercoding tree structure. That is, this means that a CU in an I slice/tilegroup, to which the separate block tree structure is applied, mayinclude a coding block of luma components or coding blocks of two chromacomponents and a CU of a P or B slice/tile group may include blocks ofthree color components (a luma component and two chroma components).Although a quadtree coding tree structure with a nested multitype treehas been described, a structure in which a CU is partitioned is notlimited thereto. For example, the BT structure and the TT structure maybe interpreted as a concept included in a multiple partitioning tree(MPT) structure, and the CU may be interpreted as being partitionedthrough the QT structure and the MPT structure. In an example where theCU is partitioned through a QT structure and an MPT structure, a syntaxelement (e.g., MPT_split_type) including information on how many blocksthe leaf node of the QT structure is partitioned into and a syntaxelement (ex. MPT_split_mode) including information on which of verticaland horizontal directions the leaf node of the QT structure ispartitioned into may be signaled to determine a partitioning structure.

In another example, the CU may be partitioned in a different way thanthe QT structure, BT structure or TT structure. That is, unlike that theCU of the lower depth is partitioned into ¼ of the CU of the higherdepth according to the QT structure, the CU of the lower depth ispartitioned into ½ of the CU of the higher depth according to the BTstructure, or the CU of the lower depth is partitioned into ¼ or ½ ofthe CU of the higher depth according to the TT structure, the CU of thelower depth may be partitioned into ⅕, ⅓, ⅜, ⅗, ⅔, or ⅝ of the CU of thehigher depth in some cases, and the method of partitioning the CU is notlimited thereto.

Overview of Inter Prediction

Hereinafter, inter prediction according to the present disclosure willbe described.

The prediction unit of an image encoding apparatus/image decodingapparatus according to the present disclosure may perform interprediction in units of blocks to derive a prediction sample. Interprediction may represent prediction derived in a manner that isdependent on data elements (e.g., sample values, motion information,etc.) of picture(s) other than a current picture. When inter predictionapplies to the current block, a predicted block (prediction block or aprediction sample array) for the current block may be derived based on areference block (reference sample array) specified by a motion vector ona reference picture indiated by a reference picture index. In this case,in order to reduce the amount of motion information transmitted in aninter prediction mode, motion information of the current block may bepredicted in units of blocks, subblocks or samples, based on correlationof motion information between a neighboring block and the current block.The motion information may include a motion vector and a referencepicture index. The motion information may further include interprediction type (L0 prediction, L1 prediction, Bi prediction, etc.)information. When applying inter prediction, the neighboring block mayinclude a spatial neighboring block present in the current picture and atemporal neighboring block present in the reference picture. A referencepicture including the reference block and a reference picture includingthe temporal neighboring block may be the same or different. Thetemporal neighboring block may be referred to as a collocated referenceblock, collocated CU (ColCU) or colBlock, and the reference pictureincluding the temporal neighboring block may be referred to as acollocated picture (colPic) or colPicture. For example, a motioninformation candidate list may be constructed based on the neighboringblocks of the current block, and flag or index information indicatingwhich candidate is selected (used) may be signaled in order to derivethe motion vector of the current block and/or the reference pictureindex.

Inter prediction may be performed based on various prediction modes. Forexample, in the case of a skip mode and a merge mode, the motioninformation of the current block may be equal to the motion informationof the selected neighboring block. In the case of the skip mode, aresidual signal may not be transmitted unlike the merge mode. In thecase of a motion information prediction (MVP) mode, the motion vector ofthe selected neighboring block may be used as a motion vector predictorand a motion vector difference may be signaled. In this case, the motionvector of the current block may be derived using a sum of the motionvector predictor and the motion vector difference. In the presentdisclosure, the MVP mode may have the same meaning as advanced motionvector prediction (AMVP).

The motion information may include L0 motion information and/or L1motion information according to the inter prediction type (L0prediction, L1 prediction, Bi prediction, etc.). The motion vector in anL0 direction may be referred to as an L0 motion vector or MVL0, and themotion vector in an L1 direction may be referred to as an L1 motionvector or MVL1 . Prediction based on the L0 motion vector may bereferred to as L0 prediction, prediction based on the L1 motion vectormay be referred to as L1 prediction, and prediction based both the L0motion vector and the L1 motion vector may be referred to as Biprediction. Here, the L0 motion vector may indicate a motion vectorassociated with a reference picture list L0 (L0) and the L1 motionvector may indicate a motion vector associated with a reference picturelist L1 (L1 ). The reference picture list L0 may include pictures beforethe current picture in output order as reference pictures, and thereference picture list L1 may include pictures after the current picturein output order. The previous pictures may be referred to as forward(reference) pictures and the subsequent pictures may be referred to asreverse (reference) pictures. The reference picture list L0 may furtherinclude pictures after the current picture in output order as referencepictures. In this case, within the reference picture list L0, theprevious pictures may be first indexed and the subsequent pictures maythen be indexed. The reference picture list L1 may further includepictures before the current picture in output order as referencepictures. In this case, within the reference picture list L1, thesubsequent pictures may be first indexed and the previous pictures maythen be indexed. Here, the output order may correspond to picture ordercount (POC) order.

FIG. 6 is a flowchart illustrating an inter prediction based video/imageencoding method.

FIG. 7 is a view illustrating the configuration of an inter predictionunit 180 according to the present disclosure.

The encoding method of FIG. 6 may be performed by the image encodingapparatus of FIG. 2. Specifically, step S610 may be performed by theinter prediction unit 180, and step S620 may be performed by theresidual processor. Specifically, step S620 may be performed by thesubtractor 115. Step S630 may be performed by the entropy encoder 190.The prediction information of step S630 may be derived by the interprediction unit 180, and the residual information of step S630 may bederived by the residual processor. The residual information isinformation on the residual samples. The residual information mayinclude information on quantized transform coefficients for the residualsamples. As described above, the residual samples may be derived astransform coefficients through the transformer 120 of the image encodingapparatus, and the transform coefficients may be derived as quantizedtransform coefficients through the quantizer 130. Information on thequantized transform coefficients may be encoded by the entropy encoder190 through a residual coding procedure.

The image encoding apparatus may perform inter prediction with respectto a current block (S610). The image encoding apparatus may derive aninter prediction mode and motion information of the current block andgenerate prediction samples of the current block. Here, inter predictionmode determination, motion information derivation and prediction samplesgeneration procedures may be simultaneously performed or any one thereofmay be performed before the other procedures. For example, as shown inFIG. 7, the inter prediction unit 180 of the image encoding apparatusmay include a prediction mode determination unit 181, a motioninformation derivation unit 182 and a prediction sample derivation unit183. The prediction mode determination unit 181 may determine theprediction mode of the current block, the motion information derivationunit 182 may derive the motion information of the current block, and theprediction sample derivation unit 183 may derive the prediction samplesof the current block. For example, the inter prediction unit 180 of theimage encoding apparatus may search for a block similar to the currentblock within a predetermined area (search area) of reference picturesthrough motion estimation, and derive a reference block whose adifference from the current block is equal to or less than apredetermined criterion or a minimum Based on this, a reference pictureindex indicating a reference picture in which the reference block islocated may be derived, and a motion vector may be derived based on aposition difference between the reference block and the current block.The image encoding apparatus may determine a mode applying to thecurrent block among various prediction modes. The image encodingapparatus may compare rate-distortion (RD) costs for the variousprediction modes and determine an optimal prediction mode of the currentblock. However, the method of determining the prediction mode of thecurrent block by the image encoding apparatus is not limited to theabove example, and various methods may be used.

For example, when a skip mode or a merge mode applies to the currentblock, the image encoding apparatus may derive merge candidates fromneighboring blocks of the current block and construct a merge candidatelist using the derived merge candidates. In addition, the image encodingapparatus may derive a reference block whose a difference from thecurrent block is equal to or less than a predetermined criterion or aminimum, among reference blocks indicated by merge candidates includedin the merge candidate list. In this case, a merge candidate associatedwith the derived reference block may be selected, and merge indexinformation indicating the selected merge candidate may be generated andsignaled to an image decoding apparatus. The motion information of thecurrent block may be derived using the motion information of theselected merge candidate.

As another example, when an MVP mode applies to the current block, theimage encoding apparatus may derive motion vector predictor (mvp)candidates from the neighboring blocks of the current block andconstruct an mvp candidate list using the derived mvp candidates. Inaddition, the image encoding apparatus may use the motion vector of themvp candidate selected from among the mvp candidates included in the mvpcandidate list as the mvp of the current block. In this case, forexample, the motion vector indicating the reference block derived by theabove-described motion estimation may be used as the motion vector ofthe current block, an mvp candidate with a motion vector having asmallest difference from the motion vector of the current block amongthe mvp candidates may be the selected mvp candidate. A motion vectordifference (MVD) which is a difference obtained by subtracting the mvpfrom the motion vector of the current block may be derived. In thiscase, index information indicating the selected mvp candidate andinformation on the MVD may be signaled to the image decoding apparatus.In addition, when applying the MVP mode, the value of the referencepicture index may be constructed as reference picture index informationand separately signaled to the image decoding apparatus.

The image encoding apparatus may derive residual samples based on theprediction samples (S620). The image encoding apparatus may derive theresidual samples through comparison between original samples of thecurrent block and the prediction samples. For example, the residualsample may be derived by subtracting a corresponding prediction samplefrom an original sample.

The image encoding apparatus may encode image information includingprediction information and residual information (S630). The imageencoding apparatus may output the encoded image information in the formof a bitstream. The prediction information may include prediction modeinformation (e.g., skip flag, merge flag or mode index, etc.) andinformation on motion information as information related to theprediction procedure. Among the prediction mode information, the skipflag indicates whether a skip mode applies to the current block, and themerge flag indicates whether the merge mode applies to the currentblock. Alternatively, the prediction mode information may indicate oneof a plurality of prediction modes, such as a mode index. When the skipflag and the merge flag are 0, it may be determined that the MVP modeapplies to the current block. The information on the motion informationmay include candidate selection information (e.g., merge index, mvp flagor mvp index) which is information for deriving a motion vector. Amongthe candidate selection information, the merge index may be signaledwhen the merge mode applies to the current block and may be informationfor selecting one of merge candidates included in a merge candidatelist. Among the candidate selection information, the mvp flag or the mvpindex may be signaled when the MVP mode applies to the current block andmay be information for selecting one of mvp candidates in an mvpcandidate list. In addition, the information on the motion informationmay include information on the above-described MVD and/or referencepicture index information. In addition, the information on the motioninformation may include information indicating whether to apply L0prediction, L1 prediction or Bi prediction. The residual information isinformation on the residual samples. The residual information mayinclude information on quantized transform coefficients for the residualsamples.

The output bitstream may be stored in a (digital) storage medium andtransmitted to the image decoding apparatus or may be transmitted to theimage decoding apparatus via a network.

As described above, the image encoding apparatus may generate areconstructed picture (a picture including reconstructed samples and areconstructed block) based on the reference samples and the residualsamples. This is for the image encoding apparatus to derive the sameprediction result as that performed by the image decoding apparatus,thereby increasing coding efficiency. Accordingly, the image encodingapparatus may store the reconstructed picture (or the reconstructedsamples and the reconstructed block) in a memory and use the same as areference picture for inter prediction. As described above, an in-loopfiltering procedure is further applicable to the reconstructed picture.

FIG. 8 is a flowchart illustrating an inter prediction based video/imagedecoding method.

FIG. 9 is a view illustrating the configuration of an inter predictionunit 260 according to the present disclosure.

The image decoding apparatus may perform operation corresponding tooperation performed by the image encoding apparatus. The image decodingapparatus may perform prediction with respect to a current block basedon received prediction information and derive prediction samples.

The decoding method of FIG. 8 may be performed by the image decodingapparatus of FIG. 3. Steps S810 to S830 may be performed by the interprediction unit 260, and the prediction information of step S810 and theresidual information of step S840 may be obtained from a bitstream bythe entropy decoder 210. The residual processor of the image decodingapparatus may derive residual samples for a current block based on theresidual information (S840). Specifically, the dequantizer 220 of theresidual processor may perform dequantization based on dequantizedtransform coefficients derived based on the residual information toderive transform coefficients, and the inverse transformer 230 of theresidual processor may perform inverse transform with respect to thetransform coefficients to derive the residual samples for the currentblock. Step S850 may be performed by the adder 235 or the reconstructor.

Specifically, the image decoding apparatus may determine the predictionmode of the current block based on the received prediction information(S810). The image decoding apparatus may determine which interprediction mode applies to the current block based on the predictionmode information in the prediction information.

For example, it may be determined whether the skip mode applies to thecurrent block based on the skip flag. In addition, it may be determinedwhether the merge mode or the MVP mode applies to the current blockbased on the merge flag. Alternatively, one of various inter predictionmode candidates may be selected based on the mode index. The interprediction mode candidates may include a skip mode, a merge mode and/oran MVP mode or may include various inter prediction modes which will bedescribed below.

The image decoding apparatus may derive the motion information of thecurrent block based on the determined inter prediction mode (S820). Forexample, when the skip mode or the merge mode applies to the currentblock, the image decoding apparatus may construct a merge candidatelist, which will be described below, and select one of merge candidatesincluded in the merge candidate list. The selection may be performedbased on the above-described candidate selection information (mergeindex). The motion information of the current block may be derived usingthe motion information of the selected merge candidate. For example, themotion information of the selected merge candidate may be used as themotion information of the current block.

As another example, when the MVP mode applies to the current block, theimage decoding apparatus may construct an mvp candidate list and use themotion vector of an mvp candidate selected from among mvp candidatesincluded in the mvp candidate list as an mvp of the current block. Theselection may be performed based on the above-described candidateselection information (mvp flag or mvp index). In this case, the MVD ofthe current block may be derived based on information on the MVD, andthe motion vector of the current block may be derived based on mvp andMVD of the current block. In addition, the reference picture index ofthe current block may be derived based on the reference picture indexinformation. A picture indicated by the reference picture index in thereference picture list of the current block may be derived as areference picture referenced for inter prediction of the current block.

The image decoding apparatus may generate prediction samples of thecurrent block based on motion information of the current block (S830).In this case, the reference picture may be derived based on thereference picture index of the current block, and the prediction samplesof the current block may be derived using the samples of the referenceblock indicated by the motion vector of the current block on thereference picture. In some cases, a prediction sample filteringprocedure may be further performed with respect to all or some of theprediction samples of the current block.

For example, as shown in FIG. 9, the inter prediction unit 260 of theimage decoding apparatus may include a prediction mode determinationunit 261, a motion information derivation unit 262 and a predictionsample derivation unit 263. In the inter prediction unit 260 of theimage decoding apparatus, the prediction mode determination unit 261 maydetermine the prediction mode of the current block based on the receivedprediction mode information, the motion information derivation unit 262may derive the motion information (a motion vector and/or a referencepicture index, etc.) of the current block based on the received motioninformation, and the prediction sample derivation unit 263 may derivethe prediction samples of the current block.

The image decoding apparatus may generate residual samples of thecurrent block based the received residual information (S840). The imagedecoding apparatus may generate the reconstructed samples of the currentblock based on the prediction samples and the residual samples andgenerate a reconstructed picture based on this (S850). Thereafter, anin-loop filtering procedure is applicable to the reconstructed pictureas described above.

As described above, the inter prediction procedure may include step ofdetermining an inter prediction mode, step of deriving motioninformation according to the determined prediction mode, and step ofperforming prediction (generating prediction samples) based on thederived motion information. The inter prediction procedure may beperformed by the image encoding apparatus and the image decodingapparatus, as described above.

Hereinafter, the step of deriving the motion information according tothe prediction mode will be described in greater detail.

As described above, inter prediction may be performed using motioninformation of a current block. An image encoding apparatus may deriveoptimal motion information of a current block through a motionestimation procedure. For example, the image encoding apparatus maysearch for a similar reference block with high correlation within apredetermined search range in the reference picture using an originalblock in an original picture for the current block in fractional pixelunit, and derive motion information using the same. Similarity of theblock may be calculated based on a sum of absolute differences (SAD)between the current block and the reference block. In this case, motioninformation may be derived based on a reference block with a smallestSAD in the search area. The derived motion information may be signaledto an image decoding apparatus according to various methods based on aninter prediction mode.

When a merge mode applies to a current block, motion information of thecurrent block is not directly transmitted and motion information of thecurrent block is derived using motion information of a neighboringblock. Accordingly, motion information of a current prediction block maybe indicated by transmitting flag information indicating that the mergemode is used and candidate selection information (e.g., a merge index)indicating which neighboring block is used as a merge candidate. In thepresent disclosure, since the current block is a unit of predictionperformance, the current block may be used as the same meaning as thecurrent prediction block, and the neighboring block may be used as thesame meaning as a neighboring prediction block.

The image encoding apparatus may search for merge candidate blocks usedto derive the motion information of the current block to perform themerge mode. For example, up to five merge candidate blocks may be used,without being limited thereto. The maximum number of merge candidateblocks may be transmitted in a slice header or a tile group header,without being limited thereto. After finding the merge candidate blocks,the image encoding apparatus may generate a merge candidate list andselect a merge candidate block with smallest RD cost as a final mergecandidate block.

The present disclosure provides various embodiments for the mergecandidate blocks configuring the merge candidate list. The mergecandidate list may use, for example, five merge candidate blocks. Forexample, four spatial merge candidates and one temporal merge candidatemay be used.

FIG. 10 is a view illustrating neighboring blocks available as a spatialmerge candidate.

FIG. 11 is a view schematically illustrating a merge candidate listconstruction method according to an example of the present disclosure.

An image encoding/decoding apparatus may insert, into a merge candidatelist, spatial merge candidates derived by searching for spatialneighboring blocks of a current block (S1110). For example, as shown inFIG. 10, the spatial neighboring blocks may include a bottom-left cornerneighboring block A₀, a left neighboring block A₁, a top-right cornerneighboring block B₀, a top neighboring block B₁, and a top-left cornerneighboring block B₂ of the current block. However, this is an exampleand, in addition to the above-described spatial neighboring blocks,additional neighboring blocks such as a right neighboring block, abottom neighboring block and a bottom-right neighboring block may befurther used as the spatial neighboring blocks. The imageencoding/decoding apparatus may detect available blocks by searching forthe spatial neighboring blocks based on priority and derive motioninformation of the detected blocks as the spatial merge candidates. Forexample, the image encoding/decoding apparatus may construct a mergecandidate list by searching for the five blocks shown in FIG. 10 inorder of A₁, B₁, B₀, A₀ and B₂ and sequentially indexing availablecandidates.

The image encoding/decoding apparatus may insert, into the mergecandidate list, a temporal merge candidate derived by searching fortemporal neighboring blocks of the current block (S1120). The temporalneighboring blocks may be located on a reference picture which isdifferent from a current picture in which the current block is located.A reference picture in which the temporal neighboring block is locatedmay be referred to as a collocated picture or a col picture. Thetemporal neighboring block may be searched for in order of abottom-right corner neighboring block and a bottom-right center block ofthe co-located block for the current block on the col picture.Meanwhile, when applying motion data compression in order to reducememory load, specific motion information may be stored as representativemotion information for each predetermined storage unit for the colpicture. In this case, motion information of all blocks in thepredetermined storage unit does not need to be stored, thereby obtainingmotion data compression effect. In this case, the predetermined storageunit may be predetermined as, for example, 16×16 sample unit or 8×8sample unit or size information of the predetermined storage unit may besignaled from the image encoding apparatus to the image decodingapparatus. When applying the motion data compression, the motioninformation of the temporal neighboring block may be replaced with therepresentative motion information of the predetermined storage unit inwhich the temporal neighboring block is located. That is, in this case,from the viewpoint of implementation, the temporal merge candidate maybe derived based on the motion information of a prediction blockcovering an arithmetic left-shifted position after an arithmetic rightshift by a predetermined value based on coordinates (top-left sampleposition) of the temporal neighboring block, not a prediction blocklocated on the coordinates of the temporal neighboring block. Forexample, when the predetermined storage unit is a 2^(n)'2^(n) sampleunit and the coordinates of the temporal neighboring block are (xTnb,yTnb), the motion information of a prediction block located at amodified position ((xTnb>>n)<<n), (yTnb>>n)<<n)) may be used for thetemporal merge candidate. Specifically, for example, when thepredetermined storage unit is a 16×16 sample unit and the coordinates ofthe temporal neighboring block are (xTnb, yTnb), the motion informationof a prediction block located at a modified position ((xTnb>>4)<<4),(yTnb>>4)<<4)) may be used for the temporal merge candidate.Alternatively, for example, when the predetermined storage unit is an8×8 sample unit and the coordinates of the temporal neighboring blockare (xTnb, yTnb), the motion information of a prediction block locatedat a modified position ((xTnb>>3)<<3), (yTnb>>3)<<3)) may be used forthe temporal merge candidate.

Referring to FIG. 11 again, the image encoding/decoding apparatus maycheck whether the number of current merge candidates is less than amaximum number of merge candidates (S1130). The maximum number of mergecandidates may be predefined or signaled from the image encodingapparatus to the image decoding apparatus. For example, the imageencoding apparatus may generate and encode information on the maximumnumber of merge candidates and transmit the encoded information to theimage decoding apparatus in the form of a bitstream. When the maximumnumber of merge candidates is satisfied, a subsequent candidate additionprocess S1140 may not be performed.

When the number of current merge candidates is less than the maximumnumber of merge candidates as a checked result of step S1130, the imageencoding/decoding apparatus may derive an additional merge candidateaccording to a predetermined method and then insert the additional mergecandidate to the merge candidate list (S1140).

When the number of current merge candidates is not less than the maximumnumber of merge candidates as a checked result of step S1130, the imageencoding/decoding apparatus may end the construction of the mergecandidate list. In this case, the image encoding apparatus may select anoptimal merge candidate from among the merge candidates configuring themerge candidate list, and signal candidate selection information (e.g.,merge index) indicating the selected merge candidate to the imagedecoding apparatus. The image decoding apparatus may select the optimalmerge candidate based on the merge candidate list and the candidateselection information.

The motion information of the selected merge candidate may be used asthe motion information of the current block, and the prediction samplesof the current block may be derived based on the motion information ofthe current block, as described above. The image encoding apparatus mayderive the residual samples of the current block based on the predictionsamples and signal residual information of the residual samples to theimage decoding apparatus. The image decoding apparatus may generatereconstructed samples based on the residual samples derived based on theresidual information and the prediction samples and generate thereconstructed picture based on the same, as described above.

When applying a skip mode to the current block, the motion informationof the current block may be derived using the same method as the case ofapplying the merge mode. However, when applying the skip mode, aresidual signal for a corresponding block is omitted and thus theprediction samples may be directly used as the reconstructed samples.

When applying an MVP mode to the current block, a motion vectorpredictor (mvp) candidate list may be generated using a motion vector ofreconstructed spatial neighboring blocks (e.g., the neighboring blocksshown in FIG. 10) and/or a motion vector corresponding to the temporalneighboring blocks (or Col blocks). That is, the motion vector of thereconstructed spatial neighboring blocks and the motion vectorcorresponding to the temporal neighboring blocks may be used as motionvector predictor candidates of the current block. When applyingbi-prediction, an mvp candidate list for L0 motion informationderivation and an mvp candidate list for L1 motion informationderivation are individually generated and used. Prediction information(or information on prediction) of the current block may includecandidate selection information (e.g., an MVP flag or an MVP index)indicating an optimal motion vector predictor candidate selected fromamong the motion vector predictor candidates included in the mvpcandidate list. In this case, a prediction unit may select a motionvector predictor of a current block from among the motion vectorpredictor candidates included in the mvp candidate list using thecandidate selection information. The prediction unit of the imageencoding apparatus may obtain and encode a motion vector difference(MVD) between the motion vector of the current block and the motionvector predictor and output the encoded MVD in the form of a bitstream.That is, the MVD may be obtained by subtracting the motion vectorpredictor from the motion vector of the current block. The predictionunit of the image decoding apparatus may obtain a motion vectordifference included in the information on prediction and derive themotion vector of the current block through addition of the motion vectordifference and the motion vector predictor. The prediction unit of theimage encoding apparatus may obtain or derive a reference picture indexindicating a reference picture from the information on prediction.

FIG. 12 is a view schematically illustrating a motion vector predictorcandidate list construction method according to an example of thepresent disclosure.

First, a spatial candidate block of a current block may be searched forand available candidate blocks may be inserted into an mvp candidatelist (S1210). Thereafter, it is determined whether the number of mvpcandidates included in the mvp candidate list is less than 2 (S1220)and, when the number of mvp candidates is two, construction of the mvpcandidate list may be completed.

In step S1220, when the number of available spatial candidate blocks isless than 2, a temporal candidate block of the current block may besearched for and available candidate blocks may be inserted into the mvpcandidate list (S1230). When the temporal candidate blocks are notavailable, a zero motion vector may be inserted into the mvp candidatelist, thereby completing construction of the mvp candidate list.

Meanwhile, when applying an mvp mode, a reference picture index may beexplicitly signaled. In this case, a reference picture index refidxL0for L0 prediction and a reference picture index refidxL1 for L1prediction may be distinguishably signaled. For example, when applyingthe MVP mode and applying Bi prediction, both information on refidxL0and information on refidxL1 may be signaled.

As described above, when applying the MVP mode, information on MVPderived by the image encoding apparatus may be signaled to the imagedecoding apparatus. Information on the MVD may include, for example, anMVD absolute value and information indicating x and y components for asign. In this case, when the MVD absolute value is greater than 0,whether the MVD absolute value is greater than 1 and informationindicating an MVD remainder may be signaled stepwise. For example,information indicating whether the MVD absolute value is greater than 1may be signaled only when a value of flag information indicating whetherthe MVD absolute value is greater than 0 is 1.

FIG. 13 is a view illustrating a syntax structure for transmitting MVDfrom an image encoding apparatus to an image decoding apparatusaccording to an embodiment of the present disclosure.

In FIG. 13, abs_mvd_greater0_flag[0] indicates whether the absolutevalue of the x component of MVD is greater than 0, andabs_mvd_greater0_flag[1] indicates the absolute value of the y componentof MVD is greater than 0. Similarly, abs_mvd_greater1_flag[0] indicateswhether the absolute value of the x component of MVD is greater than 1,and abs_mvd_greaterl_flag[1] indicates whether the absolute value of they component of MVD is greater than 1. As shown in FIG. 13,abs_mvd_greater1_flag may be transmitted only when abs_mvd_greater0_flagis 1. In FIG. 13, abs_mvd_minus2 may indicate a value obtained bysubtracting 2 from the absolute value of MVD, and mvd_sign_flag indicatewhether the sign of MVD is positive or negative. Using the syntaxstructure shown in FIG. 13, MVD may be derived as shown in Equation 1below.

MVD[compIdx]=abs_mvd_greater0_flag[compIdx]*(abs_mvd_minus2[compIdx]+2)*(1-2 * mvd_sign_flag[compIdx])   [Equation 1]

Meanwhile, MVD (MVDL0) for L0 prediction and MVD(MVDL1 ) for L1prediction may be distinguishably signaled, and the information on MVDmay include information on MVDL0 and/or information on MVDL1 . Forexample, when applying the MVP mode and applying BI prediction to thecurrent block, both the information on MVDL0 and the information onMVDL1 may be signaled.

Overview of Intra Block Copy (IBC) Prediction

Hereinafter, IBC prediction according to the present disclosure will bedescribed.

IBC prediction may be performed by a prediction unit of an imageencoding/decoding apparatus. IBC prediction may be simply referred to asIBC. The IBC may be used for content image/moving image coding such asscreen content coding (SCC). The IBC prediction may be basicallyperformed in the current picture, but may be performed similarly tointer prediction in that a reference block is derived within the currentpicture. That is, IBC may use at least one of inter predictiontechniques described in the present disclosure. For example, IBC may useat least one of the above-described motion information (motion vector)derivation methods. At least one of the inter prediction techniques maybe partially modified and used in consideration of the IBC prediction.The IBC may refer to a current picture and thus may be referred to ascurrent picture referencing (CPR).

For IBC, the image encoding apparatus may perform block matching (BM)and derive an optimal block vector (or motion vector) for a currentblock (e.g., a CU). The derived block vector (or motion vector) may besignaled to the image decoding apparatus through a bitstream using amethod similar to signaling of motion information (motion vector) in theabove-described inter prediction. The image decoding apparatus mayderive a reference block for the current block in the current picturethrough the signaled block vector (motion vector), and derive aprediction signal (predicted block or prediction samples) for thecurrent block through this. Here, the block vector (or motion vector)may indicate displacement from the current block to a reference blocklocated in an already reconstructed area in the current picture.Accordingly, the block vector (or the motion vector) may be referred toa displacement vector. Hereinafter, in IBC, the motion vector maycorrespond to the block vector or the displacement vector. The motionvector of the current block may include a motion vector (luma motionvector) for a luma component or a motion vector (chroma motion vector)for a chroma component. For example, the luma motion vector for anIBC-coded CU may be an integer sample unit (that is, integer precision).The chroma motion vector may be clipped in integer sample units. Asdescribed above, IBC may use at least one of inter predictiontechniques, and, for example, the luma motion vector may beencoded/decoded using the above-described merge mode or MVP mode.

When applying a merge mode to the luma IBC block, a merge candidate listfor the luma IBC block may be constructed similarly to a merge candidatelist in the inter mode described with reference to FIG. 11. However, inthe case of the luma IBC block, a temporal neighboring block may not beused as a merge candidate.

When applying the MVP mode to the luma IBC block, an mvp candidate listfor the luma IBC block may be constructed similarly to the mvp candidatelist in the inter mode described with reference to FIG. 12. However, inthe case of the luma IBC block, a temporal candidate block may not beused as the mvp candidate.

In IBC, a reference block is derived from the already reconstructed areain the current picture. In this case, in order to reduce memoryconsumption and complexity of the image decoding apparatus, only apredefined area among already reconstructed areas in the current picturemay be referenced. The predefined area may include a current CTU inwhich the current block is included. By restricting referenceablereconstructed area to the predefined area, the IBC mode may beimplemented in hardware using a local on-chip memory.

The image encoding apparatus for performing IBC may search thepredefined area to determine a reference block with smallest RD cost andderive a motion vector (block vector) based on the positions of thereference block and the current block.

Whether to apply IBC to the current block may be signaled as IBCperformance information at a CU level. Information on a signaling method(IBC MVP mode or IBC skip/merger mode) of the motion vector of thecurrent block may be signaled. IBC performance information may be usedto determine the prediction mode of the current block. Accordingly, theIBC performance information may be included in information on theprediction mode of the current block.

In the case of the IBC skip/merge mode, a merge candidate index may besignaled to indicate a block vector to be used for prediction of thecurrent luma block among block vectors included in the merge candidatelist. In this case, the merge candidate list may include IBC-encodedneighboring blocks. The merge candidate list may be configured toinclude spatial merge candidates and not to include temporal mergecandidates. In addition, the merge candidate list may further includehistory-based motion vector predictor (HMVP) candidates and/or pairwisecandidates.

In the case of the IBC MVP mode, a block vector difference value may beencoded using the same method as a motion vector difference value of theabove-described inter mode. The block vector prediction method mayconstruct and use an mvp candidate list including two candidates aspredictors similarly to the MVP mode of the inter mode. One of the twocandidates may be derived from a left neighboring block and the othercandidate may be derived from a top neighboring block. In this case,only when the left or top neighboring block is encoded in IBC,candidates may be derived from the corresponding neighboring block. Ifthe left or top neighboring block is not available, for example, is notencoded in IBC, a default block vector may be included in the mvpcandidate list as a predictor. In addition, information (e.g., flag)indicating one of two block vector predictors is signaled and used ascandidate selection information similarly to the MVP mode of the intermode. The mvp candidate list may include an HMVP candidate and/or a zeromotion vector as the default block vector.

The HMVP candidate may be referred to as a history-based MVP candidate,and an MVP candidate used before encoding/decoding of the current block,a merge candidate or a block vector candidate may be stored in an HMVPlist as HMVP candidates. Thereafter, when the merge candidate list ofthe current block or the mvp candidate list does not include a maximumnumber of candidates, candidates stored in the HMVP list may be added tothe merge candidate list or mvp candidate list of the current block asHMVP candidates

The pairwise candidate means a candidate derived by selecting twocandidates according to a predetermined order from among candidatesalready included in the merge candidate list of the current block andaveraging the selected two candidates.

FIG. 14 is a flowchart illustrating an IBC based video/image encodingmethod.

FIG. 15 is a view illustrating the configuration of a prediction unitfor performing an IBC based video/image encoding method according to thepresent disclosure.

The encoding method of FIG. 14 may be performed by the image encodingapparatus of FIG. 2. Specifically, step S1410 may be performed by theprediction unit and step S1420 may be performed by the residualprocessor. Specifically, step S1420 may be performed by the subtractor115. Step S1430 may be performed by the entropy encoder 190. Theprediction information of step S1430 may be derived by the predictionunit and the residual information of step S1430 may be derived by theresidual processor. The residual information is information on theresidual samples. The residual information may include information onquantized transform coefficients for the residual samples. As describedabove, the residual samples may be derived by the transform coefficientsthrough the transformer 120 of the image encoding apparatus, and thetransform coefficients may be derived by transform coefficientsquantized through the quantizer 130. Information on the quantizedtransform coefficients may be encoded by the entropy encoder 190 througha residual coding procedure.

The image encoding apparatus may perform IBC prediction (IBC basedprediction) for the current block (S1410). The image encoding apparatusmay derive a prediction mode and motion vector (block vector) of thecurrent block and generate prediction samples of the current block. Theprediction mode may include at least one of the above-described interprediction modes. Here, prediction mode determination, motion vectorderivation and prediction samples generation procedures may besimultaneously performed or any one procedure may be performed beforethe other procedures. For example, as shown in FIG. 15, the predictionunit of the image encoding apparatus for performing an IBC-basedvideo/image encoding method may include a prediction mode determinationunit, a motion vector derivation unit and a prediction sample derivationunit. The prediction mode determination unit may determine theprediction mode of the current block, the motion vector derivation unitmay derives the motion vector of the current block, and the predictionsample derivation unit may derive the prediction samples of the currentblock. For example, the prediction unit of the image encoding apparatusmay search for a block similar to the current block in a reconstructedarea (or a certain area (search area) of the reconstructed area) of acurrent picture and derive a reference block whose a difference from thecurrent block is equal to or less than a certain criterion or a minimum.The image encoding apparatus may derive a motion vector based on adisplacement difference between the reference block and the currentblock. The image encoding apparatus may determine a mode applying to thecurrent block among various prediction modes. The image encodingapparatus may compare RD costs for the various prediction modes anddetermine an optimal prediction mode for the current block. However, amethod of determining the prediction mode for the current block by theimage encoding apparatus is not limited to the above example and variousmethods may be used.

For example, when applying a skip mode or a merge mode to the currentblock, the image encoding apparatus may derive merge candidates fromneighboring blocks of the current block and construct a merge candidatelist using the derived merge candidates. In addition, the image encodingapparatus may derive a reference block whose a difference from thecurrent block is equal to or less than a certain criterion or a minimumamong reference blocks indicated by the merge candidates included in themerge candidate list. In this case, a merge candidate associated withthe derived reference block may be selected and merge index informationindicating the selected merge candidate may be generated and signaled tothe image decoding apparatus. Using the motion vector of the selectedmerge candidate, the motion vector of the current block may be derived.

As another example, when applying an MVP mode to the current block, theimage encoding apparatus may derive motion vector predictor (mvp)candidates from the neighboring blocks of the current block andconstruct an mvp candidate list using the derived mvp candidates. Inaddition, the image encoding apparatus may use the motion vector of themvp candidate selected from among the mvp candidates included in the mvpcandidate list as the mvp of the current block. In this case, forexample, a motion vector indicating a reference block derived by theabove-described motion estimation may be used as the motion vector ofthe current block, and an mvp candidate having a smallest differencefrom the motion vector of the current block among the mvp candidates maybecome the selected mvp candidate. A motion vector difference (MVD)which is obtained by subtracting the mvp from the motion vector of thecurrent block may be derived. In this case, index information indicatingthe selected mvp candidate and information on the MVD may be signaled tothe image decoding apparatus.

The image encoding apparatus may derive residual samples based on theprediction samples (S1420). The image encoding apparatus may derive theresidual samples through comparison between the original samples of thecurrent block and the prediction samples. For example, the residualsample may be derived by subtracting the corresponding prediction samplefrom the original sample.

The image encoding apparatus may encode image information includingprediction information and residual information (S1430). The imageencoding apparatus may output the encoded image information in the formof a bitstream. The prediction information may include prediction modeinformation (e.g., skip flag, merge flag or mode index) and informationon a motion vector as information related to the prediction procedure.Among the prediction mode information, the skip flag indicates whetherto apply the skip mode to the current block and the merge flag indicateswhether to apply the merge mode to the current block. Alternatively, theprediction mode information may indicate one of a plurality ofprediction modes, such as a mode index. When the skip flag and the mergeflag are 0, it may be determined that the MVP mode applies to thecurrent block. The information on the motion vector may includecandidate selection information (e.g., merge index, mvp flag or mvpindex) which is information for deriving the motion vector. Among thecandidate selection information, the merge index may be signaled whenapplying the merge mode to the current block and may be information forselecting one of the merge candidates included in the merge candidatelist. Among the candidate selection information, the mvp flag or mvpindex may be signaled when applying the MVP mode to the current blockand may be information for selecting one of the mvp candidates includedin the mvp candidate list. In addition, the information on the motionvector may include information on the above-described MVD. In addition,the information on the motion vector may include information indicatingwhether to apply L0 prediction, L1 prediction or bi prediction. Theresidual information is information on the residual samples. Theresidual information may include information on the quantized transformcoefficients for the residual samples.

The output bitstream may be stored in a (digital) storage medium andtransmitted to the image decoding apparatus or may be transmitted to theimage decoding apparatus through a network.

Meanwhile, as described above, the image encoding apparatus may generatea reconstructed picture (picture including reconstructed samples and areconstructed block) based on the reference samples and the residualsamples. This is for the image encoding apparatus to derive the sameprediction result as that performed by the image decoding apparatus,thereby increasing coding efficiency. Accordingly, the image encodingapparatus may store the reconstructed picture (or reconstructed samplesand reconstructed block) in a memory and use the same as a referencepicture for inter prediction. An in-loop filtering procedure is furtherapplicable to the reconstructed picture, as described above.

FIG. 16 is a flowchart illustrating an IBC based video/image decodingmethod.

FIG. 17 is a view illustrating a configuration of a prediction unit forperforming an IBC based video/image decoding method according to thepresent disclosure.

The image decoding apparatus may perform operation corresponding tooperation performed by the image encoding apparatus. The image decodingapparatus may perform IBC prediction for a current block based onreceived prediction information to derive prediction samples.

The decoding method of FIG. 16 may be performed by the image decodingapparatus of FIG. 3. Steps S1610 to S1630 may be performed by theprediction unit and the prediction information of step S1610 and theresidual information of step S1640 may be obtained from a bitstream bythe entropy decoder 210. The residual processor of the image decodingapparatus may derive residual samples for the current block based on theresidual information (S1640). Specifically, the dequantizer 220 of theresidual processor may perform dequantization based on the quantizedtransform coefficient derived based on the residual information toderive transform coefficients, and the inverse transformer 230 of theresidual processor may perform inverse transform with respect to thetransform coefficients to derive residual samples for the current block. Step S1650 may be performed by the adder 235 or the reconstructor.

Specifically, the image decoding apparatus may determine the predictionmode of the current block based on the received prediction information(S1610). The image decoding apparatus may determine which predictionmode applies to the current block based on the prediction modeinformation in the prediction information.

For example, it may be determined whether to apply the skip mode to thecurrent block based on the skip flag. In addition, it may be determinedwhether to apply the merge node or MVP mode to the current block basedon the merge flag. Alternatively, one of various prediction modecandidates may be selected based on the mode index. The prediction modecandidates may include a skip mode, a merge mode and/or an MVP mode ormay include the above-described various inter prediction modes.

The image encoding apparatus may derive the motion vector of the currentblock based on the determined prediction mode (S1620). For example, whenapplying the skip mode or the merge mode to the current block, the imagedecoding apparatus may construct the above-described merge candidatelist and select one of the merge modes included in the merge candidatelist. The selection may be performed based on the above-describedcandidate selection information (merge index). The motion vector of thecurrent block may be derived using the motion vector of the selectedmerge candidate. For example, the motion vector of the selected mergecandidate may be used as the motion vector of the current block.

As another example, when applying the MVP mode to the current block, theimage decoding apparatus may construct an mvp candidate list and use themotion vector of the mvp candidate selected from among the mvpcandidates included in the mvp candidate list as the mvp of the currentblock. The selection may be performed based on the above-describedcandidate selection information (mvp flag or mvp index). In this case,the MVD of the current block may be derived based on information on theMVD, and the motion vector of the current block may be derived based onthe mvp and MVD of the current block.

The image decoding apparatus may generate prediction samples of thecurrent block based on the motion vector of the current block (S1630).The prediction samples of the current block may be derived using thesamples of the reference block indicated by the motion vector of thecurrent block on the current picture. In some cases, a prediction samplefiltering procedure for all or some of the prediction samples of thecurrent block may be further performed.

For example, as shown in FIG. 17, the prediction unit of the imagedecoding apparatus for performing an IBC based video/image decodingmethod may include a prediction mode determination unit, a motion vectorderivation unit and a prediction sample derivation unit. The predictionunit of the image decoding apparatus may determine the prediction modefor the current block based on the received prediction mode informationin the prediction mode determination unit, derive the motion vector ofthe current block based on the received information on the motion vectorin the motion vector derivation unit , and derive the prediction samplesof the current block in the prediction sample derivation unit.

The image decoding apparatus may generate residual samples of thecurrent block based on the received residual information (S1640). Theimage decoding apparatus may generate reconstructed samples for thecurrent block based on the prediction samples and the residual samples,and generate a reconstructed picture based on this (S1650). Thereafter,an in-loop filtering procedure is further applicable to thereconstructed picture, as described above.

As described above, one unit (e.g., a coding unit (CU)) may include aluma block (luma coding block (CB)) and a chroma block (chroma CB). Inthis case, the luma block and the chroma block corresponding thereto mayhave the same motion information (e.g., motion vector) or differentmotion information. For example, the motion information of the chromablock may be derived based on the motion information of the luma block,such that the luma block and the chroma block corresponding thereto havethe same motion information.

Hereinafter, a method of deriving motion information of a IBC-encodedchroma block (chroma IBC block) from a corresponding luma block will bedescribed.

As described above, a luma block and a chroma block in one CTU may bepartitioned into the same block tree structures SINGLE_TREE or separateblock tree structures DUAL_TREE.

In case of a single tree, a luma block and a chroma block correspondingthereto use the same prediction mode. That is, a luma blockcorresponding to a chroma IBC block is encoded in IBC. Accordingly,motion information of the chroma IBC block may be derived based on themotion information of the corresponding luma block.

In case of a dual tree, a luma block and a chroma block correspondingthereto may be encoded in the same prediction mode or may be encoded indifferent prediction modes. That is, a luma block corresponding to achroma IBC block may be encoded in IBC or may be encoded in anotherprediction mode.

FIG. 18 is a flowchart illustrating a conventional method of derivingmotion information of a current chroma IBC block based on acorresponding luma block.

When a current chroma IBC block is input (S1800), it may be determinedwhether a tree structure of a current block is a dual tree (S1810). Whenthe tree structure of the current block is a single tree, a luma blockcorresponding to the chroma IBC block is also encoded in IBC.Accordingly, motion information of the corresponding luma block may beobtained (S1820) and, using this, motion information of the chroma IBCblock may be derived (S1830).

In step S1810, when the tree structure of the current block is a dualtree, the luma block corresponding to the chroma IBC block may beencoded in IBC or may be encoded in a mode (e.g., an intra mode) otherthan IBC. In this case, in order to derive the motion information of thechroma IBC block, the luma block corresponding to the chroma IBC blockmay be partitioned into a plurality of luma subblocks (e.g., 4×4 blocks)(S1840) and motion information of the chroma subblock corresponding toeach of the plurality of luma subblocks may be derived. Specifically, itmay be determined whether a first luma subblock of the plurality of lumasubblocks is a block encoded in IBC (S1850). When the luma subblock hasbeen encoded in IBC, motion information of the corresponding lumasubblock may be obtained (S1860), and, based on this, motion informationof the chroma subblock may be derived (S1870). Thereafter, it may bedetermined whether the current subblock is a last subblock (S1880), and,when the current subblock is not a last subblock, a next subblock may beinput (S1890) and steps S1850 to S1880 may be repeated. In step S1880,when the current subblock is a last subblock, a process of deriving themotion information of the chroma IBC block may be finished.

FIG. 19 is a view illustrating a luma block corresponding to a chromaIBC block in case of a dual tree.

In FIG. 19, a white region indicates a prediction block encoded in IBCand a gray region indicates a prediction block encoded in an intra mode.In addition, in FIG. 19, a dotted line indicates a partitioning linepartitioning the luma block into the plurality of luma subblocks, asdescribed in step S1840.

In the example shown in FIG. 19, the corresponding luma block may bepartitioned into a plurality of prediction blocks, and each predictionblock may be encoded in IBC or intra mode. According to a conventionalmethod described with reference to FIG. 18, in order to derive themotion information of the chroma IBC block, the corresponding luma blockmay be partitioned into a plurality of luma subblocks having apredetermined size (4×4) and motion information of the correspondingchroma subblock may be derived based on each of the plurality of lumasubblocks. In this case, for example, in the example shown in FIG. 19,luma subblock 0 has been encoded in IBC and thus includes motioninformation of IBC, and motion information of the chroma subblock at acorresponding position may be derived based on motion information ofluma subblock 0. However, luma subblock 1 has been encoded in the intramode and thus does not include motion information of IBC. Accordingly,motion information of the chroma subblock at a position corresponding toluma subblock 1 may not be derived.

According to the conventional method described with reference to FIGS.18 and 19, when the corresponding luma subblock is encoded in a modeother than IBC, motion information of the chroma subblock may not beappropriately derived and thus efficiency of IBC for the correspondingchroma subblock may be reduced.

Various embodiments of the present disclosure provide a method ofsolving a problem that there is no motion information of a correspondingluma block when a chroma block is encoded/decoded in IBC and thus motioninformation of a chroma IBC block cannot be derived.

FIG. 20 is a flowchart illustrating an embodiment of the presentdisclosure of deriving motion information of a current chroma IBC blockbased on a corresponding luma block.

In an embodiment of the present disclosure shown in FIG. 20, when thereis no motion information of a corresponding luma block, motioninformation of a chroma IBC block is derived based on default motioninformation instead of the motion information of the corresponding lumablock.

In FIG. 20, when a current chroma IBC block is input (S2000), it may bedetermined whether a tree structure of a current block is a dual tree(S2010). When the tree structure of the current block is a single tree,a luma block corresponding to the chroma IBC block is also encoded inIBC. Accordingly, motion information of the corresponding luma block maybe obtained (S2020) and, using this, motion information of the chromaIBC block may be derived (S2030).

In step S2010, when the tree structure of the current block is a dualtree, the luma block corresponding to the chroma IBC block may beencoded in IBC or may be encoded in a mode (e.g., an intra mode) otherthan IBC. In this case, in order to derive the motion information of thechroma IBC block, the luma block corresponding to the chroma IBC blockmay be partitioned into a plurality of luma subblocks (e.g., 4×4subblocks) (S2040) and motion information of the chroma subblockcorresponding to each of the plurality of luma subblocks may be derived.Specifically, it may be determined whether a first luma subblock of theplurality of luma subblocks has been encoded in IBC (S2050). When theluma subblock is a block encoded in IBC, motion information of thecorresponding luma subblock may be obtained (S2060), and, based on this,motion information of the chroma subblock may be derived (S2070).Thereafter, it may be determined whether the current subblock is a lastsubblock (S2080), and, when the current subblock is not a last subblock,a next subblock may be input (S2090) and steps S2050 to S2080 may berepeated. In step S2080, when the current subblock is a last subblock, aprocess of deriving the motion information of the chroma IBC block maybe finished.

According to the present disclosure, in step S2050, when the lumasubblock is a block which is not encoded in IBC, the motion informationof the chroma subblock may be derived based on default motioninformation defaultMv (S2075). That is, when there is no motioninformation of a corresponding luma subblock, the motion information ofthe chroma subblock may be derived based on the default motioninformation.

The default motion information may be predefined between an imageencoding apparatus and an image decoding apparatus and may be equallyderived in the image encoding apparatus and the image decoding apparatusaccording to a predetermined procedure.

According to the present disclosure, even if there is no motioninformation of a corresponding luma block, since the motion informationof the chroma IBC block may be derived based on the default motioninformation, the above-described problems of the conventional method maybe solved.

As an example of the default motion information of FIG. 20, since a unitfor deriving motion information is a subblock, default motioninformation may be determined based on a subblock size subBlockSize. Forexample, the default motion information may be determined to be(-subBlockSize, 0) or (0, -subBlockSize). In the example of FIG. 20, aluma subblock size may be set to 4×4 and the default motion informationmay be determined to be a chroma subblock size corresponding to the lumasubblock size. As another example of the default motion information ofFIG. 20, default motion information may be determined based on a widthcbWidth or height cbHeight of a current chroma IBC block. For example,the default motion information may be determined to be (-cbWidth, 0) or(0, -cbHeight).

The default motion information of FIG. 20 may be derived from acorresponding luma block. For example, the default motion informationmay be derived based on motion information at a predetermined positionof the corresponding luma block.

FIG. 21 is a view illustrating a predetermined position of acorresponding luma block for deriving default motion information.

As shown in FIG. 21, the predetermined position may be a center positionof the corresponding luma block (S2110). For example, the default motioninformation may be determined to be motion information at the centerposition of the corresponding luma block, as shown in Equation 2 below.

defaultMv=mvL[xCb+(cbWidth>>1)][yCb+(cbHeight>>1)]  [Equation 2]

In Equation 2 above, (xCb, yCb) denotes the coordinates of a top-leftsample of the corresponding luma block, and cbWidth and cbHeightrespectively denote the width and height of the corresponding lumablock. Accordingly, (xCb+(cbWidth>>1), yCb+(cbHeight>>1)) denotes thecoordinates of the center position of the corresponding luma block.mvL[x][y] may denote motion information of a luma block at a (x, y)position.

Alternatively, as shown in FIG. 21, the predetermined position may be atop-left position 2120 of the corresponding luma block. For example, thedefault motion information may be determined to be motion information atthe top-left position of the corresponding luma block, as shown inEquation 3 below.

defaultMv=mvL[xCb][yCb]  [Equation 3]

However, the predetermined position for deriving the default motioninformation is not limited to the above-described center position ortop-left position and another position in the corresponding luma blockmay be used.

As a modification, a plurality of available motion information may beobtained from a corresponding luma block and a statistical value of theplurality of obtained motion information may be determined as thedefault motion information. For example, an average value of N pieces ofmotion information obtained from the corresponding luma block may bedetermined as the default motion information. As the statistical value,a maximum value, a minimum value or a median value may be used, insteadof the average value.

Alternatively, the default motion information of FIG. 20 may be derivedbased on the motion information at the predetermined position of thecorresponding luma block, and, when there is no motion information atthe predetermined position, predetermined motion information may be usedas default motion information.

FIG. 22 is a flowchart illustrating an example of deriving defaultmotion information according to the present disclosure.

As described above, default motion information may be derived based onmotion information at a predetermined position of a corresponding lumablock. To this end, in step S2210, the predetermined position forderiving the default motion information may be derived. For example,when the predetermined position is a center position of thecorresponding luma block, the coordinates of the center position may bederived as shown in Equation 2 above.

When the predetermined position is derived, it may be determined whetherthe corresponding luma block at the corresponding position has beenencoded in IBC (S2220). When the predetermined position of thecorresponding luma block has not been encoded in IBC, default motioninformation may not be derived from the corresponding luma block.Accordingly, in this case, predetermined motion information may bedetermined as default motion information (S2230). In this case, thepredetermined motion information may be determined based on a subblocksize or a size (width or height) of a current block, as described above.

In step S2220, when the predetermined position of the corresponding lumablock has been encoded in IBC, the default motion information may bedetermined to be motion information at a corresponding position of thecorresponding luma block (S2240).

According to another embodiment of the present disclosure, only when thedefault motion information may be derived from the corresponding lumablock, IBC for the chroma block is limited to be available.

FIG. 23 is a flowchart illustrating a method of encoding a currentchroma block depending on whether default motion information is able tobe derived from a corresponding luma block.

First, when a current chroma block is input (S2310), an image encodingapparatus may determine whether default motion information is able to bederived from a corresponding luma block (S2320). For example, asdescribed with reference to FIGS. 20 and 21, the default motioninformation may be derived based on motion information at apredetermined position of the corresponding luma block. For example,when the predetermined position is a center position, determination ofstep S2320 may be made according to the following conditional equation.

if(CuPredMode[x0+(CbWidth>>1)]][y0+(CbHeight>>1)]==MODE_IBC)  [Conditional Equation]

When the center position of the corresponding luma block is encoded inIBC, it may be determined that the above conditional equation issatisfied and the default motion information is able to be derived fromthe corresponding luma block. When the center position of thecorresponding luma block is encoded in a mode other than IBC, it may bedetermined that the above conditional equation is not satisfied and thedefault motion information is not able to be derived from thecorresponding luma block.

In step S2320, upon determining that the default motion information isnot able to be derived from the corresponding luma block, the imageencoding apparatus may determine a prediction mode for the input chromablock except for an IBC mode. That is, when the default motioninformation is not able to be derived from the corresponding luma block,the chroma block may be limited not to be encoded in IBC. In this case,information specifying whether the chroma block has been encoded in IBC(IBC performance information, e.g., pred_mode_jbc_flag) may not beexplicitly signaled.

In step S2320, upon determining that the default motion information isable to be derived from the corresponding luma block, an image encodingapparatus may determine a prediction mode for the input chroma blockincluding an IBC mode. That is, when the default motion information isable to be derived from the corresponding luma block, the current chromablock may be encoded in the IBC mode. Accordingly, the image encodingapparatus may determine one of various prediction modes including an IBCmode as a prediction mode for a current chroma block. In this case, IBCperformance information specifying whether the chroma block has beenencoded in IBC may be included in a bitstream and explicitly signaled.

FIG. 24 is a flowchart illustrating a method of decoding a currentchroma block depending on whether default motion information is able tobe derived from a corresponding luma block.

First, when a current chroma block is input (S2410), an image decodingapparatus may determine whether default motion information is able to bederived from a corresponding luma block (S2420). Step 52420 is equal tostep S2320 of FIG. 23 and thus a detailed description thereof will beomitted.

In step S2420, upon determining that the default motion information isnot able to be derived from the corresponding luma block, IBCperformance information is not explicitly signaled. Accordingly, theimage decoding apparatus may not decode the IBC performance informationand determine that IBC is not performed. The image decoding apparatusmay perform prediction and/or decoding with respect to the currentchroma block in a mode other than the IBC mode (S2430).

In step S2420, upon determining that the default motion information isable to be derived from the corresponding luma block, the image decodingapparatus may obtain and decode IBC performance information from abitstream (S2440). It may be determined whether the prediction mode ofthe current chroma block is IBC based on the IBC performance (S2450),and, in case of IBC, the current chroma block may be predicted and/ordecoded in the IBC mode (S2460). When the prediction mode of the currentchroma block is not IBC, the image decoding apparatus may performprediction and/decoding with respect to the current chroma block in amode other than the IBC mode (S2430).

In the examples described with reference to FIGS. 23 and 24, the defaultmotion information may be derived from motion information of anotherposition (e.g., a top-left position) instead of the center position ofthe corresponding luma block. That is, the predetermined position of thecorresponding luma block for deriving the default motion information isnot limited to the center position and any other position may be used.

In the examples described with reference to FIGS. 20 to 24, the defaultmotion information may be derived in units of the corresponding lumablocks. The derived default motion information is commonly applicable toall subblocks included in the corresponding luma block in step S2075.

FIG. 25 is a flowchart illustrating an embodiment of the presentdisclosure of deriving motion information of a current block based on acorresponding block.

First, a current block to be encoded/decoded may be input (S2510). Thecurrent block may be a block encoded/decoded in IBC, and a motion vectorof the current block may not be directly signaled and may be derivedfrom motion information of a corresponding block. For example, thecurrent block may be a chroma IBC block. When the current block isinput, a corresponding block corresponding to the current block may bespecified and whether motion information of the corresponding block isavailable may be determined (S2520). For example, the correspondingblock may be a corresponding luma block corresponding to a chroma IBCblock. Step S2520 may be performed based on a determination as towhether the corresponding block has been encoded in IBC. Alternatively,step S2520 may be performed based on a determination as to whether thecorresponding block has motion information. For example, when thecorresponding block has been encoded in IBC or has motion information,it may be determined that motion information of the corresponding blockis available. Otherwise, it may be determined that motion information ofthe corresponding block is not available.

Although the current block and the corresponding block are a chromablock and a corresponding luma block in the above description, thepresent disclosure is not limited thereto. For example, the currentblock may be a luma block and the corresponding block may be acorresponding chroma block. Alternatively, the current block may be afirst color component block and the corresponding block may be a secondcolor component block. In this case, the first color component may be,for example, one of a luma component and a plurality of chromacomponents, and the second color component may be a color componentdifferent from the first color component. For example, the first colorcomponent block may be a first chroma component block and the secondcolor component block may be a second chroma component block.

Referring to FIG. 25 again, in step S2520, when the motion informationof the corresponding block is available, motion information of thecorresponding block may be obtained (S2530), and motion information ofthe current block may be derived based on the motion information of thecorresponding block (S2540). In step S2520, when the motion informationof the corresponding block is not available, default motion informationmay be determined (S2550) and the motion information of the currentblock may be derived based on the default motion information (S2560). Asa method of determining the default motion information of step S2550, atleast one of the methods described with reference to FIGS. 20 to 24 or acombination of two or more thereof may be used.

According to the present disclosure, when motion information of acurrent block is derived from motion information of a correspondingblock, even if there is no motion information of a corresponding block,the motion information of the current block may be derived based ondefault motion information. Therefore, it is possible to efficientlyencode/decode the current block when there is no motion information of acorresponding block.

While the exemplary methods of the present disclosure described aboveare represented as a series of operations for clarity of description, itis not intended to limit the order in which the steps are performed, andthe steps may be performed simultaneously or in different order asnecessary. In order to implement the method according to the presentdisclosure, the described steps may further include other steps, mayinclude remaining steps except for some of the steps, or may includeother additional steps except for some steps.

In the present disclosure, the image encoding apparatus or the imagedecoding apparatus that performs a predetermined operation (step) mayperform an operation (step) of confirming an execution condition orsituation of the corresponding operation (step). For example, if it isdescribed that predetermined operation is performed when a predeterminedcondition is satisfied, the image encoding apparatus or the imagedecoding apparatus may perform the predetermined operation afterdetermining whether the predetermined condition is satisfied.

The various embodiments of the present disclosure are not a list of allpossible combinations and are intended to describe representativeaspects of the present disclosure, and the matters described in thevarious embodiments may be applied independently or in combination oftwo or more.

Various embodiments of the present disclosure may be implemented inhardware, firmware, software, or a combination thereof. In the case ofimplementing the present disclosure by hardware, the present disclosurecan be implemented with application specific integrated circuits(ASICs), Digital signal processors (DSPs), digital signal processingdevices (DSPDs), programmable logic devices (PLDs), field programmablegate arrays (FPGAs), general processors, controllers, microcontrollers,microprocessors, etc.

In addition, the image decoding apparatus and the image encodingapparatus, to which the embodiments of the present disclosure areapplied, may be included in a multimedia broadcasting transmission andreception device, a mobile communication terminal, a home cinema videodevice, a digital cinema video device, a surveillance camera, a videochat device, a real time communication device such as videocommunication, a mobile streaming device, a storage medium, a camcorder,a video on demand (VoD) service providing device, an OTT video (over thetop video) device, an Internet streaming service providing device, athree-dimensional (3D) video device, a video telephony video device, amedical video device, and the like, and may be used to process videosignals or data signals. For example, the OTT video devices may includea game console, a blu-ray player, an Internet access TV, a home theatersystem, a smartphone, a tablet PC, a digital video recorder (DVR), orthe like.

FIG. 26 is a view showing a contents streaming system, to which anembodiment of the present disclosure is applicable.

As shown in FIG. 26, the contents streaming system, to which theembodiment of the present disclosure is applied, may largely include anencoding server, a streaming server, a web server, a media storage, auser device, and a multimedia input device.

The encoding server compresses contents input from multimedia inputdevices such as a smartphone, a camera, a camcorder, etc. into digitaldata to generate a bitstream and transmits the bitstream to thestreaming server. As another example, when the multimedia input devicessuch as smartphones, cameras, camcorders, etc. directly generate abitstream, the encoding server may be omitted.

The bitstream may be generated by an image encoding method or an imageencoding apparatus, to which the embodiment of the present disclosure isapplied, and the streaming server may temporarily store the bitstream inthe process of transmitting or receiving the bitstream.

The streaming server transmits the multimedia data to the user devicebased on a user's request through the web server, and the web serverserves as a medium for informing the user of a service. When the userrequests a desired service from the web server, the web server maydeliver it to a streaming server, and the streaming server may transmitmultimedia data to the user. In this case, the contents streaming systemmay include a separate control server. In this case, the control serverserves to control a command/response between devices in the contentsstreaming system.

The streaming server may receive contents from a media storage and/or anencoding server. For example, when the contents are received from theencoding server, the contents may be received in real time. In thiscase, in order to provide a smooth streaming service, the streamingserver may store the bitstream for a predetermined time.

Examples of the user device may include a mobile phone, a smartphone, alaptop computer, a digital broadcasting terminal, a personal digitalassistant (PDA), a portable multimedia player (PMP), navigation, a slatePC, tablet PCs, ultrabooks, wearable devices (e.g., smartwatches, smartglasses, head mounted displays), digital TVs, desktops computer, digitalsignage, and the like.

Each server in the contents streaming system may be operated as adistributed server, in which case data received from each server may bedistributed.

The scope of the disclosure includes software or machine-executablecommands (e.g., an operating system, an application, firmware, aprogram, etc.) for enabling operations according to the methods ofvarious embodiments to be executed on an apparatus or a computer, anon-transitory computer-readable medium having such software or commandsstored thereon and executable on the apparatus or the computer.

Industrial Applicability

The embodiments of the present disclosure may be used to encode ordecode an image.

1. An image decoding method performed by an image decoding apparatus,the image decoding method comprising: determining whether a predictionmode of a current block is an intra block copy (IBC) mode based oninformation on the prediction mode of the current block; based on theprediction mode of the current block being an IBC mode, deriving motioninformation of the current block based on motion information of acorresponding block corresponding to the current block; and generating aprediction block for the current block, by performing IBC predictionbased on the derived motion information of the current block, wherein,based on absence of motion information of the corresponding block, themotion information of the current block is derived based on defaultmotion information.
 2. The image decoding method of claim 1, wherein thecurrent block is a chroma block and the corresponding block is a lumablock, wherein the deriving the motion information of the current blockcomprises: partitioning the corresponding luma block into luma subblockshaving a predetermined size; and for each of the luma subblocks,determining whether the luma subblock is a block encoded in IBC and,based on this, deriving motion information of a chroma subblockcorresponding to the luma subblock.
 3. The image decoding method ofclaim 2, wherein, based on the luma subblock being a block encoded inIBC, the motion information of the corresponding chroma subblock isderived based on motion information of the luma subblock, and wherein,based on the luma subblock being not a block encoded in IBC, the motioninformation of the corresponding chroma subblock is derived based on thedefault motion information.
 4. The image decoding method of claim 2,wherein the default motion information is derived based on apredetermined size of the subblock or a size of the current chromablock.
 5. The image decoding method of claim 1, wherein the defaultmotion information is derived based on motion information at apredetermined position of the corresponding block.
 6. The image decodingmethod of claim 5, wherein the predetermined position of thecorresponding block is a center position or a top-left position of thecorresponding block.
 7. The image decoding method of claim 1, furthercomprising deriving the default motion information, wherein the derivingthe default motion information comprises: deriving a predeterminedposition of the corresponding block; determining whether thepredetermined position of the corresponding block has been encoded inIBC; based on the predetermined position of the corresponding blockbeing encoded in IBC, deriving the default motion information based onmotion information of the predetermined position of the correspondingblock; and based on the predetermined position of the correspondingblock being not encoded in IBC, deriving the default motion informationbased on predetermined motion information.
 8. The image decoding methodof claim 1, wherein whether the prediction mode of the current block isan IBC mode is determined based on whether a predetermined position ofthe corresponding block has been encoded in IBC.
 9. The image decodingmethod of claim 8, wherein, based on the predetermined position of thecorresponding block being encoded in IBC, the prediction mode of thecurrent block is determined based on IBC performance informationobtained from a bitstream.
 10. The image decoding method of claim 8,wherein, based on the predetermined position of the corresponding blockbeing not encoded in IBC, the prediction mode of the current block isdetermined to be a mode other than the IBC mode.
 11. An image decodingapparatus, comprising: a memory; and at least one processor, wherein theat least one processor is configured to: determine whether a predictionmode of a current block is an intra block copy (IBC) mode based oninformation on the prediction mode of the current block; based on theprediction mode of the current block being an IBC mode, derive motioninformation of the current block based on motion information of acorresponding block corresponding to the current block; and generate aprediction block for the current block, by performing IBC predictionbased on the derived motion information of the current block, wherein,based on absence of motion information of the corresponding block, themotion information of the current block is derived based on defaultmotion information.
 12. An image encoding method performed by an imageencoding apparatus, the image encoding method comprising: determiningwhether a prediction mode of a current block is an intra block copy(IBC) mode; based on the prediction mode of the current block being anIBC mode, deriving motion information of the current block based onmotion information of a corresponding block corresponding to the currentblock; and generating a prediction block for the current block, byperforming IBC prediction based on the derived motion information of thecurrent block, wherein, based on absence of motion information of thecorresponding block, the motion information of the current block isderived based on default motion information.
 13. The image encodingmethod of claim 12, wherein the current block is a chroma block and thecorresponding block is a luma block, wherein the deriving the motioninformation of the current block comprises: partitioning thecorresponding luma block into luma subblocks having a predeterminedsize; and for each of the luma subblocks, determining whether the lumasubblock is a block encoded in IBC and, based on this, deriving motioninformation of a chroma subblock corresponding to the luma subblock. 14.The image encoding method of claim 13, wherein, based on the lumasubblock being a block encoded in IBC, the motion information of thecorresponding chroma subblock is derived based on motion information ofthe luma subblock, and wherein, based on the luma subblock being not ablock encoded in IBC, the motion information of the corresponding chromasubblock is derived based on the default motion information.
 15. Amethod of transmitting a bitstream generated by the image encodingmethod of claim 14.